I need to identify and prove a theorem which can be proved in Euclidean geometry but not non-Euclidean. I need to have a diagram and explain why it works in Euclidean, but not in non-Euclidean.

I'm having trouble wrapping my head around this. It has been a long time since I've done this sort of thing! I know the difference between the two is the parallel postulate, the only Theorem that comes to mind is the Pythagorean Theorem, am I on the right track?

THank you for your help!