Lucid dreaming is when you're dreaming, but you're aware of it. Theoretically, you could get to a point where you can control you the dream. It's a tricky process, but to help it along, Will Finucane built a pair of lucidity-inducing goggles.

He rigged a pair of LEDs to a circuit, and programmed them to blink two hours after he had fallen asleep. By this point he would be in REM state, so he'd be (hopefully) dreaming. The little lights would flash, but only light enough to bring him to awareness, in order to consciously manipulate the world he had created in his head, rather than to rouse him.

Incept Yourself With Lucid Dreaming Goggles

Though they're not supposed to wake you, it might be difficult to go to sleep in the first place wearing giant, obstructive eyewear. Plus, if you're a light sleeper, even very dim lights might cause you to stir. But if you're curious, you can try it yourself. Finucane posted a how-to online. It's like self-inception! [Mad Science via Hackaday via Geekosystem]