Light travels at roughly 300,000 kilometres per second or 186,000 miles per second.
It can travel from Earth to the Moon in just over one second.
Imagine you're on a space ship which is starting to accelerate towards the speed of light.
Time will slow down, although to you it won't seem like it, but when you stop, everyone else will be older than you.
It might sound unlikely, but back in 1971 scientists proved it. They took two very accurate clocks and put one on board a jumbo jet.
After flying for a couple of hours, the plane landed and the two clocks were held next to each other.
The jumbo's clock was a tiny bit behind the ground clock! Moving even quite slowly had made a measurable difference!