Pass by reference vs pass by value

This is illustrating a tricky distinction: a reference is a POINTER to a value. It's not its own thing.

If you see there, the second example has the first cup not filling up.
That’s a bit more helpful

Still don't really understand the difference. Isn't the command fillCup() the same?

This is just a small piece of a bigger script. The stuff before the fillCup() affects how it reacts. This is just an infograph to help visualise it