What happens when you pass a value to a function

This connects memory to code you write every day. When you pass a number to a function, the function gets its own copy. When you pass a list or object in Python, JavaScript, Java or C#, the function gets a copy of the reference, so both names point to the same heap object.

This is sometimes called pass by sharing, and two names for one object is called aliasing. Appending inside the function changes the caller's list. But assigning a new list to the parameter does not affect the caller at all.

C++ copies the whole object by default, unless you pass a reference or a pointer. Knowing which case applies explains most "my data changed" and "my change disappeared" bugs. It is also one of the things the milestone asks you to write up.