Functions.

Seventh class. If you copy a formula into three places, a bug means fixing it three times. Functions solve that.

def name(parameters): creates a recipe. return sends a result back to the call site. Calling calculate_bmi(weight, height) runs the recipe with concrete values.

Today you build one clear function: calculate_bmi. It receives weight and height and sends the result back with return. Calling code stores that result and prints it. This highlights an important distinction: return gives a value to the program; print only displays text to a person.

Mini example:

def double(n):
    return n * 2
print(double(5))

Common mistake: printing inside the function instead of returning, or forgetting to call the function.

Note: today we only learn functions (def / return). Python modules and import come in a later chapter.