In Solitude, there is a kid named Kayd who runs around playing tag with his friends and occasionally taunting me. If I cast any spell in front of him, he approaches me to ask if I can make him invisible, and I can convince him that he is. But however much fun it is watching him running around thinking he's invisible, it would be much more fun to see him running around actually invisible.
I don't have a Thief character advanced enough to reverse-pickpocket him with an invisibility potion, but if I did, would he use it?