Cascarones have been a tradition of Fiesta since long before I was a child. Made of hollowed eggs filled with confetti, they are cracked on other people's heads as a fun (and often surprising) gesture. They are (usually) a fantastic and glorious treat for both 'cracker' and 'recipient.'
For such a small object, the cascarone evokes a huge affect. They are usually around when some sort of celebration is going on, so times are happy.
In San Antonio, cascarones are readily available on street corners, in shops and in local grocery stores starting around Easter time and going through Fiesta (the end of April). If you aren't lucky enough to be in South Texas during that time, you can easily make your own.
I'm sorry that I don't have visual directions to accompany this post, but the steps are very straightforward (albeit a bit messy). If you don't have access to a prepacked egg dye kit, you can easily make your own egg dye. See below for what else you'll need to get started.