diff --git a/13_object_oriented_programming_advanced.ipynb b/13_object_oriented_programming_advanced.ipynb
index 91842885..a498eb09 100644
--- a/13_object_oriented_programming_advanced.ipynb
+++ b/13_object_oriented_programming_advanced.ipynb
@@ -23,6 +23,7 @@
     "    - [`super()`](#super())\n",
     "    - [Quiz on Inheritance](#Quiz-on-Inheritance)\n",
     "    - [Exercise: Triathlete](#Exercise:-Triathlete)\n",
+    "  - [The `__new__` method](#The-__new__-method)\n",
     "  - [Abstract Classes](#Abstract-Classes)\n",
     "    - [Quiz on Abstraction](#Quiz-on-Abstraction)\n",
     "    - [Exercise: Banking System](#Exercise:-Banking-System)\n",
@@ -554,6 +555,66 @@
    "cell_type": "markdown",
    "id": "39",
    "metadata": {},
+   "source": [
+    "## The `__new__` method\n",
+    "\n",
+    "Let's also take a look at the `__new__` method, as explained [here](https://www.pythontutorial.net/python-oop/python-__new__/).\n",
+    "A more technical explanation is given in the [official documentation](https://docs.python.org/3/reference/datamodel.html#object.__new__).\n",
+    "\n",
+    "`__new__` is a static method of a class.\n",
+    "When you create a new instance of a class, Python actually calls `__new__` first, to create the object, and then calls `__init__` to initialize the object's attributes.\n",
+    "Override the `__new__` method if you want to tweak the object at creation time.\n",
+    "\n",
+    "The signature of `__new__` is:\n",
+    "```python\n",
+    "def __new__(cls, *args, **kwargs):\n",
+    "    instance = super().__new__(cls)\n",
+    "    return instance\n",
+    "```\n",
+    "\n",
+    "- `cls`: The class being instantiated (analogous to `self` in `__init__`, but refers to the class itself, not yet an instance). This is passed automatically by Python.\n",
+    "- `*args` / `**kwargs`: Any positional or keyword arguments that were passed to the class constructor. The same ones that will later be forwarded to `__init__`.\n",
+    "- `super().__new__(cls)`: Delegates the actual object allocation to the parent class (ultimately `object`, the base class of anything in Python). This creates and returns the new instance in memory.\n",
+    "- The method should the new instance. If it returns an instance of `cls`, Python will then call `__init__` on it.\n",
+    "\n",
+    "A common real-world use case is implementing the [**Singleton pattern**](https://www.geeksforgeeks.org/python/singleton-pattern-in-python-a-complete-guide/), ensuring that only one instance of a class ever exists.\n",
+    "By controlling object creation inside `__new__` you can return the same instance every time the class is called."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "40",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "class Singleton:\n",
+    "    _instance = None  # class-level variable to hold the single instance\n",
+    "\n",
+    "    def __new__(cls, *args, **kwargs):\n",
+    "        if cls._instance is None:\n",
+    "            print(\"Creating new instance...\")\n",
+    "            cls._instance = super().__new__(cls)\n",
+    "        else:\n",
+    "            print(\"Returning existing instance.\")\n",
+    "        return cls._instance\n",
+    "\n",
+    "    def __init__(self, value):\n",
+    "        self.value = value\n",
+    "\n",
+    "\n",
+    "a = Singleton(42)\n",
+    "b = Singleton(99)\n",
+    "\n",
+    "print(f\"a.value = {a.value}\")\n",
+    "print(f\"b.value = {b.value}\")\n",
+    "print(f\"a is b: {a is b}\")  # True because both variables point to the same object"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "41",
+   "metadata": {},
    "source": [
     "## Abstract Classes\n",
     "\n",
@@ -570,7 +631,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "40",
+   "id": "42",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -589,7 +650,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "41",
+   "id": "43",
    "metadata": {},
    "source": [
     "We mark methods of an abstract class as **virtual** – meaning that they must be overridden by each subclass that inherits from the abstract parent – using the `abstractmethod()` decorator.\n",
@@ -601,7 +662,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "42",
+   "id": "44",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -610,7 +671,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "43",
+   "id": "45",
    "metadata": {},
    "source": [
     "Let's create two concrete subclasses of `Shape`:"
@@ -619,7 +680,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "44",
+   "id": "46",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -648,7 +709,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "45",
+   "id": "47",
    "metadata": {},
    "source": [
     "Now we are allowed to create instances of the subclasses and also call their methods:"
@@ -657,7 +718,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "46",
+   "id": "48",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -672,7 +733,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "47",
+   "id": "49",
    "metadata": {},
    "source": [
     "### Quiz on Abstraction"
@@ -681,7 +742,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "48",
+   "id": "50",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -692,7 +753,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "49",
+   "id": "51",
    "metadata": {},
    "source": [
     "### Exercise: Banking System\n",
@@ -743,7 +804,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "50",
+   "id": "52",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -753,7 +814,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "51",
+   "id": "53",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -779,7 +840,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "52",
+   "id": "54",
    "metadata": {},
    "source": [
     "## Decorators\n",
@@ -792,7 +853,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "53",
+   "id": "55",
    "metadata": {},
    "source": [
     "### @classmethod\n",
@@ -814,7 +875,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "54",
+   "id": "56",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -856,7 +917,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "55",
+   "id": "57",
    "metadata": {},
    "source": [
     "### @staticmethod\n",
@@ -871,7 +932,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "56",
+   "id": "58",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -893,7 +954,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "57",
+   "id": "59",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -913,7 +974,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "58",
+   "id": "60",
    "metadata": {},
    "source": [
     "### @property\n",
@@ -930,7 +991,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "59",
+   "id": "61",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -953,7 +1014,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "60",
+   "id": "62",
    "metadata": {},
    "source": [
     "We can access the `area` property, just like any other class attribute.\n",
@@ -963,7 +1024,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "61",
+   "id": "63",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -972,7 +1033,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "62",
+   "id": "64",
    "metadata": {},
    "source": [
     "### Setters & Getters\n",
@@ -990,7 +1051,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "63",
+   "id": "65",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1016,7 +1077,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "64",
+   "id": "66",
    "metadata": {},
    "source": [
     "Create an instance and use the getter:"
@@ -1025,7 +1086,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "65",
+   "id": "67",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1035,7 +1096,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "66",
+   "id": "68",
    "metadata": {},
    "source": [
     "Update the radius using the setter:"
@@ -1044,7 +1105,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "67",
+   "id": "69",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1055,7 +1116,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "68",
+   "id": "70",
    "metadata": {},
    "source": [
     "What happens when we enter an invalid value?"
@@ -1064,7 +1125,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "69",
+   "id": "71",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1073,7 +1134,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "70",
+   "id": "72",
    "metadata": {},
    "source": [
     "Finally let's use the deleter:"
@@ -1082,7 +1143,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "71",
+   "id": "73",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1091,7 +1152,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "72",
+   "id": "74",
    "metadata": {},
    "source": [
     "We are no longer able to access the deleted attribute:"
@@ -1100,7 +1161,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "73",
+   "id": "75",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1109,7 +1170,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "74",
+   "id": "76",
    "metadata": {},
    "source": [
     "### Quiz on Decorators"
@@ -1118,7 +1179,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "75",
+   "id": "77",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1129,7 +1190,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "76",
+   "id": "78",
    "metadata": {},
    "source": [
     "## Encapsulation\n",
@@ -1150,7 +1211,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "77",
+   "id": "79",
    "metadata": {},
    "source": [
     "### Public\n",
@@ -1159,7 +1220,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "78",
+   "id": "80",
    "metadata": {},
    "source": [
     "### Private\n",
@@ -1171,7 +1232,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "79",
+   "id": "81",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1190,7 +1251,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "80",
+   "id": "82",
    "metadata": {},
    "source": [
     "### Protected\n",
@@ -1203,7 +1264,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "81",
+   "id": "83",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1222,7 +1283,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "82",
+   "id": "84",
    "metadata": {},
    "source": [
     "### Quiz on Encapsulation"
@@ -1231,7 +1292,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "83",
+   "id": "85",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1242,7 +1303,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "84",
+   "id": "86",
    "metadata": {},
    "source": [
     "## How to write better classes\n",
@@ -1252,7 +1313,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "85",
+   "id": "87",
    "metadata": {},
    "source": [
     "### Using `dataclasses`\n",
@@ -1263,7 +1324,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "86",
+   "id": "88",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1276,7 +1337,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "87",
+   "id": "89",
    "metadata": {},
    "source": [
     "A simpler way, however, would be to import `dataclass` from the `dataclasses` module.\n",
@@ -1287,7 +1348,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "88",
+   "id": "90",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1303,7 +1364,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "89",
+   "id": "91",
    "metadata": {},
    "source": [
     "Now, with the use of these auto generated methods, we can create an instance of the class and print a representation of the object, without any additional code.\n",
@@ -1313,7 +1374,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "90",
+   "id": "92",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1327,7 +1388,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "91",
+   "id": "93",
    "metadata": {},
    "source": [
     "### Using `attrs`\n",
@@ -1349,7 +1410,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "92",
+   "id": "94",
    "metadata": {},
    "source": [
     "Let's rewrite the previous example, this time using `attrs`.\n",
@@ -1359,7 +1420,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "93",
+   "id": "95",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1382,7 +1443,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "94",
+   "id": "96",
    "metadata": {},
    "source": [
     "However, `attrs` also provides **validators**.\n",
@@ -1394,7 +1455,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "95",
+   "id": "97",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1418,7 +1479,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "96",
+   "id": "98",
    "metadata": {},
    "source": [
     "### Quiz on `attrs` and `dataclasses`"
@@ -1427,7 +1488,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "97",
+   "id": "99",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1438,7 +1499,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "98",
+   "id": "100",
    "metadata": {},
    "source": [
     "## Exercises"
@@ -1447,7 +1508,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "99",
+   "id": "101",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1456,7 +1517,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "100",
+   "id": "102",
    "metadata": {},
    "source": [
     "### Store Inventory\n",
@@ -1496,7 +1557,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "101",
+   "id": "103",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1532,7 +1593,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "102",
+   "id": "104",
    "metadata": {},
    "source": [
     "### Music Streaming Service\n",
@@ -1565,7 +1626,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "103",
+   "id": "105",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -1615,13 +1676,13 @@
   },
   {
    "cell_type": "markdown",
-   "id": "104",
+   "id": "106",
    "metadata": {},
    "source": []
   },
   {
    "cell_type": "markdown",
-   "id": "105",
+   "id": "107",
    "metadata": {},
    "source": [
     "### The N-body problem\n",
@@ -1648,7 +1709,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "106",
+   "id": "108",
    "metadata": {},
    "source": [
     "<div class=\"alert alert-block alert-info\">\n",
@@ -1661,7 +1722,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "107",
+   "id": "109",
    "metadata": {
     "lines_to_next_cell": 2,
     "tags": []
@@ -1675,7 +1736,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "108",
+   "id": "110",
    "metadata": {},
    "source": [
     "<div class=\"alert alert-block alert-warning\">\n",
@@ -1686,7 +1747,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "109",
+   "id": "111",
    "metadata": {},
    "source": [
     "Each of the strings output by your solution function below should be something like\n",
@@ -1700,7 +1761,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "110",
+   "id": "112",
    "metadata": {
     "lines_to_next_cell": 2,
     "tags": []
@@ -1714,7 +1775,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "111",
+   "id": "113",
    "metadata": {},
    "source": [
     "<div class=\"alert alert-block alert-danger\">\n",
@@ -1741,7 +1802,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "112",
+   "id": "114",
    "metadata": {},
    "source": [
     "To have a complete account of the moons' orbits, you need to compute the **total energy of the system**.\n",
@@ -1784,7 +1845,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "113",
+   "id": "115",
    "metadata": {},
    "source": [
     "<div class=\"alert alert-block alert-info\">\n",
@@ -1797,7 +1858,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "114",
+   "id": "116",
    "metadata": {
     "lines_to_next_cell": 2,
     "tags": []
@@ -1811,7 +1872,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "115",
+   "id": "117",
    "metadata": {},
    "source": [
     "<div class=\"alert alert-block alert-info\">\n",
@@ -1824,7 +1885,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "116",
+   "id": "118",
    "metadata": {},
    "source": [
     "Here's an example of the input format:\n",
@@ -1839,7 +1900,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "117",
+   "id": "119",
    "metadata": {},
    "source": [
     "<div class=\"alert alert-block alert-warning\">\n",
@@ -1851,7 +1912,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "118",
+   "id": "120",
    "metadata": {
     "tags": []
    },
@@ -1906,7 +1967,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.12.10"
+   "version": "3.12.11"
   }
  },
  "nbformat": 4,