ELI5: Relativity

Relativity means that time, speed, and motion can look different depending on who is watching and how they are moving.

• If you are sitting still and your friend is running, you see your friend moving, but your friend sees you moving too. That is why motion is relative: it depends on the observer.
• Einstein said the speed of light is always the same for everyone, even if they are moving.
• Because of that, very fast things can act strange: clocks can seem slower, and moving things can look a little shorter.
• In everyday life, you do not notice these effects because they only matter when something moves super fast, almost as fast as light.
• Another part of relativity says gravity is like a heavy ball making a stretchy blanket dip down. Smaller things roll toward it, which is a simple way to picture why planets go around the Sun.