Go outside and you will be showered, every day, with proof of the theory of special relativity. This proof comes in the form of muons, so you won't actually see it, but we'll tell you why the presence of these particles proves Einstein right.
Relativity is best known for giving us a new view of time. The faster you go, the slower time moves. There are other effects as well. For example, the faster you go, the more squished you get.
Einstein's theory of relativity says that time passes more slowly the closer you get to the ground, because you're closer to Earth's gravitational field. It's a tiny difference, but big enough for a pair of laser-powered clocks to actually measure.