One of us hides our eyes and then slowly reveals them, which causes peals of laughter from a baby. This game is called “peekaboo”.
An early theory of why babies enjoy peekaboo is that they are surprised when things come back after being out of sight. The game isn't just a joke, but helps babies test and re-test a fundamental principle of existence: that things stick around even when you can't see them.
As the baby gets older they can enjoy the adult hiding and reappearing, but after a year or so they can learn to take control by hiding and reappearing themselves.