Why should you not put a magnet near a TV or computer?

Magnets can only harm televisions with a display tube in them. This is because tube televisions use an electron beam to "paint" the image on the backside of the screen. This electron beam excites phosphors which give of light. There are three different types of phosphors, one each for red, green, and blue.

When a magnet is placed too close to a tube television, the electron beam will be bent and cause the image to distort, typically by changing the colors on the screen. If the magnet is left close to the television for too long, it will magnetize the metal within the tube and the distortions will be present even if the magnet is removed.

Modern flat panel televisions, such as plasma and LCD types, don't suffer from this problem.

In the case of computers, the magnet may change the data stored on the hard drive, which uses extremely small magnets to read and write data to a spinning metal disc. If the magnet changes the data stored on the disc, there's no way to recover it.