diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..600d2d3
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+.vscode
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diff --git a/chap9_classes.md b/chap9_classes.md
index dc3db37..7c055e8 100644
--- a/chap9_classes.md
+++ b/chap9_classes.md
@@ -130,5 +130,78 @@ Use **inheritance** when the class is a specialized version of another class.
 
 Creating a `__init__()` of a child class:
 
-1. Call `__init__()` from parent class, to initialize parent's attributes
-2. 
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+1. Call `__init__()` from parent class use `super().__init__()`, to initialize parent's attributes (as shown below)
+2. Add specialized attributes
+
+```python
+class ElectricCar(Car):
+    """Represent aspects of a car, specific to electric vehicles."""
+
+    def __init__(self, make, model, year):
+        """Initialize attributes of the parent class."""
+        super().__init__(make, model, year)
+```
+
+* `super()` is a special function that allows us to call a method from parentclass. It will initialize all inherited attributes for ElectricCar
+
+### Defining Attributes and Methods for the Child Class
+
+Add attributes/methods for child class
+
+```python
+class ElectricCar(Car):
+    """Represent aspects of a car, specific to electric vehicles."""
+
+    def __init__(self, make, model, year):
+        """
+        Initialize attributes of the parent class.
+        Then initialize attributes specific to an electric car.
+        """
+        super().__init__(make, model, year)
+        self.battery_size = 75
+
+    def describe_battery(self):
+        """Print a statement describing the battery size."""
+        print(f"This car has a {self.battery_size}-kWh battery.")
+```
+
+* `self.battery_size` is attribute unique to ElectricCar, not for any parent class
+* You can add attribute/method for child class as many as you want.
+
+### Overriding Methods from Parent Class
+
+Python can disregard parent class's method and only use redefined method in child class
+
+```python
+class Car:
+    """A simple attempt to represent a car."""
+
+    def __init__(self, make, model, year):
+        """Initialize attributes to describe a car."""
+        ...
+        self.gas_tank = 0
+
+    ...
+
+    def fill_gas_tank(self, gas):
+        """Add gas into tank"""
+        self.gas_tank = self.gas_tank + gas
+
+class ElectricCar(Car):
+    """Represent aspects of a car, specific to electric vehicles."""
+    
+    ...
+
+    def fill_gas_tank(self):
+        """Electric cars don't have gas tanks"""
+        print("This car doesn't need gas tank!")
+```
+
+* `fill_gas_tank()` override parent method, with argument list changed
+
+### Instance as Attributes
+
+Sometimes we can use an instance of an class (object) as an attribute of another class.
+
+e.g. After grouping multiple all battery related attributes/method into a new class
+
diff --git a/src/car.py b/src/car.py
index dad4990..e1f1a7d 100644
--- a/src/car.py
+++ b/src/car.py
@@ -7,6 +7,7 @@ class Car:
         self.model = model
         self.year = year
         self.odometer_reading = 0
+        self.gas_tank = 0
 
     def get_descriptive_name(self):
         """Return a neatly formatted descriptive name."""
@@ -21,7 +22,33 @@ class Car:
         """Set the odometer reading to the given value."""
         self.odometer_reading = mileage
 
+    def fill_gas_tank(self, gas):
+        """Add gas into tank"""
+        self.gas_tank = self.gas_tank + gas
+
+class ElectricCar(Car):
+    """Represent aspects of a car, specific to electric vehicles."""
+
+    def __init__(self, make, model, year):
+        """
+        Initialize attributes of the parent class.
+        Then initialize attributes specific to an electric car.
+        """
+        super().__init__(make, model, year)
+        self.battery_size = 75
+
+    def describe_battery(self):
+        """Print a statement describing the battery size."""
+        print(f"This car has a {self.battery_size}-kWh battery.")
+
+    def fill_gas_tank(self):
+        """Electric cars don't have gas tanks"""
+        print("This car doesn't need gas tank!")
+
 my_new_car = Car('audi', 'a4', 2019)
 print(my_new_car.get_descriptive_name())
 my_new_car.update_odometer(23)
-my_new_car.read_odometer()
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+my_new_car.read_odometer()
+
+my_tesla = ElectricCar('telsa', 'model s', 2019)
+print(my_tesla.get_descriptive_name())
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