A syringe that can do anything to you but mostly/always inflates people so they get fat and round. But it could do a gender swap. Turn you into solid gold. Turn you into an animal. Turn into a shapeshifter.
Turn you into your favorite character. You can turn into slime. You can forget everything. You can turn into Spamton from Deltarune. You can turn into a god. The possibilities are endless. Most/all of the time, it inflates people so and they get fat and round