UV-Erase

many chips have a clear glass window in the top of them. If you expose them to short UV (i.e. the kind that causes instant cancer) from a sterlizer lamp or UV-eraser bulb for sufficient time, the information stored on the chip is erased and it can be re-used.

Note: "blacklights" that hippies like to use in their rooms are not reliable for erasing chips.

Note #2: sunlight is not reliable for erasing chips.

Note #3: this link explains how to build an eraser.