The file
An egg-shaped keychain with a pet inside that needed feeding, cleaning and attention, and would die in a day if it didn't get them. Younger Gen X had them in high school; their kids have them now.
The schools-banning-them story is real: teachers confiscated them and the pets died in the desk drawer. That was the first time a toy made a whole generation feel guilt.
Sources: Wikipedia — Tamagotchi