This is actually a much deeper question than it appears on the surface. The simple answer most textbooks give is technically correct but misses a lot of nuance. Let me try to explain it in a way that captures the full picture.

Great question! The math behind this is complex but the concept is actually straightforward once you see it the right way. Forget what you learned in school for a moment and think about it from first principles.

I'd recommend the book 'A Brief History of Time' by Stephen Hawking, or for the math side, 'The Joy of x.' Your library has them. They're written for curious regular people. You don't need a degree to understand the big ideas.