Time dilation

Time dilation is like a trick that happens when you move fast. Imagine it like this: if you and your friend both have stopwatches and start counting off the seconds together, it looks like your watches are going at the same speed. But if you jump on a really fast roller coaster, it's like your friend's watch slows down. So when you get off the roller coaster, your watch will say that less time has passed than it does for your friend. That's time dilation!