What if you kept squishing an object—like the rabbit on this page—into a smaller and smaller space? You'd want to pull
your hand away fast, because soon the rabbit would create a kind of sinkhole, and nothing, including light, could get out once it entered. Everything—including the rabbit—would fall in and never
come out again!
These bottomless dimples in space are called black
holes. Before Einstein there were predictions that there might be something like black holes, but his General Theory of Relativity helped people to understand
years later how they exist.