This unique stamp was issued to celebrate the total solar eclipse that crossed the United States on August 21, 2017. What makes this stamp particularly strange is its use of thermochromic ink. When the stamp is at room temperature, it shows a black circle representing the eclipsed sun. However, when a finger or other source of heat is pressed on the black circle, the ink becomes transparent, revealing an image of the full moon. This interactive feature was a first for the U.S. Postal Service and was designed to simulate the experience of the eclipse. The back of the stamp pane also features a map of the eclipse's path across the country. The stamp was issued in Laramie, Wyoming, and was extremely popular, selling out in many post offices.