Due to Pluto's strange atmosphere, it had Giant Snowflakes, each about the size of a hula hoop. As they don't melt in Earth's atmosphere, they're likely composed of a substance other than water. In 1958, Superman took some of these back to Earth, where he let some school kids play with them. Afterward, he left them in a time capsule by the Metropolis Museum. ("The Secret of the Space Souvenirs")