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.