If you've got a remote of any kind (TV, AC, whatever) around you, take a quick look at the top. There's a little bulb like thingamajig there you've probably noticed like a million times before but never really given a thought to, since it doesn't really do much. Or does it? *cue suspense music*
Now press a button, any button, on the remote and look at it through your phone camera, or any digital camera, and boom! We got light folks! Right from that very bulb-like thingy that was so very staid only moments before.
This little bulb is actually called an IR emitter, and the light it emits is generally not visible. However, it is easily visible through digital cameras. This doesn't mean that it's just an easy trick for cheap thrills however, as you can actually use it to figure out if your remote is dying or dead. If the light the remote emits is dim, the battery is weak, and if there's no light whatsoever, you got a dead remote yo!
And that's all there is to it!