When I was learning the Python Crash Course, one question that is not answered in the book is to use an Instance as Attributes.

Let me show a quick example:

```
class Car():
    def __init__(self, make, model):
        self.make = make
        self.model = model
        self.tank = Tank()

    def get_car_info(self):
        long_name = self.make + ' ' + self.model
        print('The car is', long_name)

class Tank():
    def __init__(self, tank_size=20):
        self.tank_size = tank_size
    def tank_info(self):
        print('The tank size is', self.tank_size, 'gallons')

my_car = Car('Audi', 'S4')
my_car.get_car_info()
my_car.tank.tank_info()

>>>
The car is Audi S4
The tank size is 20 gallons

```

Apparently, I created two classes named Car and Tank, I want to use the Tank class instance as an attribute for the Car. The codes will work out, no problem here.

But what I noticed is that the code that I invoke the Tank method:

```
my_car.tank.tank_info()

```

If I understand correctly, the first 'tank' should mean the class 'Tank'. So why should I lowercase it? I tried to upper case, the code won't work.