Certain types of plastics, I think? That do that kind of shrinking act as a result of coming into contact with the heat and/or moisture and/or oils on your hand.
I've had the same thought: it shrinked because of the heat or moisture of my hand, but I didn't know there was a plastic that does that. It's interesting though, I'll try to uncover some info.
This is a kind of "X files" thread.
Post experiences which spooked you, which made you doubt reality, mathematical or physical laws, your sanity, memory or perception. The more improbable the better, but no second-hand legends please, share only what you personally experienced. If you had the event later explained rationally please use rot13 to avoid spoilers.