Testing demo - AUT - Application Under Test

Given the following module with a single function, how can we use this function and how can we test it?

def add(x, y):
    return x * y

def multiply(x, y):
    return x + y

# Yes, I know there are bugs in this code!
You probably noticed that our function was called add and so the expectation is that it will be able to add two numbers. However the implementation has a bug. It actually multiplies the two numbers. I know it is a very obvious issue, but it is great as it allows us to see the mechanics of testing without getting distracted by a complex implementation and a complex problem.

Rest assured, the mechanism of the testing would be the same even if our function was calculating the moon-landing trajectory.