Cascaron is the Spanish word for eggshell. A cascarone is a chicken egg that has been cracked at one end, emptied, and washed out. The eggshell is then brightly painted or dyed. When it is dry, it is filled with confetti and a piece of tissue paper is glued over the hole.
It's fun to make cascarones, although you can also buy them. But the best part is cracking them over someone's head! And the person who has a cascarone smashed over his head is supposed to have good luck!