In Portal, you control a gun that can blast two connected oval portals on different surfaces -- floors, ceilings and walls. If you step through the first portal, you emerge immediately from the other, teleported instantaneously through space, as if you walked through a magic mirror.
This quickly leads any curious player to try a little stunt: You put a portal on the floor in front of you, and then one on the ceiling directly above it. Step into the first hole, and you instantly fall out of the hole in the ceiling -- whereupon you fall back into the hole on the ground. Woo hoo! You are now falling endlessly through the holes, over and over again, in a dreamlike, self-created infinite loop.