The Conveyor Belt Secret: A Dad, a Son, and an Unseen Moment at the Supermarket

He was standing at the supermarket checkout. At the next checkout, a dad and his young son approach the cashier.

When the father starts to pay, the son tilts his head, sticks out his tongue, and licks the conveyor belt that was moving at that moment. Apparently, he decided to look at the long wet trail his tongue left.
When his dad looked at him, the boy had already stopped licking the belt and no one but us saw it.