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.

This is something I struggled with until a professor explained it using an analogy that finally clicked. The real world is messy and doesn't always fit neatly into the simplified models we learn in textbooks.

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.

Carl Sagan's 'Cosmos' explains stuff like this better than any textbook. If you can find the old TV series or the book, watch or read it. He had a gift for making the universe make sense without dumbing it down. Highly recommend.