Rabbit droppings
Johnny is on his grandpa's farm in the rabbit enclosure.
The ground is covered in rabbit droppings. Johnny asks, "What are all of the pellets on the floor grandpa?"
His grandpa replies, "Oh those? Those are smart pills. You eat them and you get smarter."
Johnny likes the sound of that so he grabs a large handful of them and shoves them into his mouth, "Yuck! Grandpa, these taste like crap."
His grandpa replies, "You're getting smarter already."