Don't feel bad for not getting this right away. Some of the smartest people in history spent their whole lives on questions like this. The fact that you're curious enough to ask puts you ahead of most people. Keep asking why.

I'm a physics grad student and this is one of my favorite questions to explain. The key insight is that our everyday intuition doesn't always apply at extreme scales — very small (quantum), very large (cosmological), or very fast (relativistic).

Scientists love this kind of question because it's how real discoveries start. 'That's funny...' is supposedly how a lot of breakthroughs begin. Keep that curiosity — it's worth more than memorizing facts for a test.

Be careful about 'common sense' here — a lot of science is counterintuitive. The whole point of doing experiments is that the universe often doesn't work the way our gut tells us it should. The Earth feels flat and stationary, after all.