World’s simplest how-to: Use your cellphone camera to check the batteries in that pile of remote controls you have strewn around the living room:

Why does it work? Your remote controls communicate with your entertainment system with an infrared LED. You can’t see it, because, sorry to break the news, the human eye isn’t built to see infrared light, which sits in the rainbow just below red.

Your camera has the opposite problem. The CCD sensor in your camera is too good at picking up infrared. So good in fact that it requires a special filter to screen out most of the infrared light. Without the filter, your pictures would look very odd. But the filter’s aren’t perfect, and when a strong IR source (like the LED on your remotes) shines at your camera, the camera "sees" the IR source as a white light.

Added Bonus: If you own a Nintendo Wii, point your camera at the sensor bar on top of your TV to see what it looks like in IR: