The honest answer is that scientists don't fully understand this yet. We have good models that make accurate predictions, but the 'why' underneath is still an active area of research. Anyone who tells you they have the complete answer is oversimplifying.

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.

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.

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.

There's a great explanation of this on the HowStuffWorks website, and the NASA site has good material too if it's space-related. Both are written for normal people, not scientists, so you won't get lost in jargon.