The Total Eclipse of the Sun stamp is the first U.S. stamp to use thermochromic ink, which reacts to the heat of your touch. Placing your finger over the black disc on the stamp causes the ink to change from black to clear to reveal an underlying image of the moon [see scan]. The image reverts back to the black disc once it cools. The thermochromic effect diminishes with time and exposure to UV light, so keep them out of direct sunlight.