The art of taking a backpack, removing all items from the inside, turning it inside out, placing the items back in, along with the straps, and then closing it. The final result closely resembles a turtle shell. The key to this act is timing, and when the person who has been turtled later comes across his/her bag, the look on his/her face will tell you that you have succeeded. This act can be performed purely for the sake of entertainment, or as an act of revenge.
Wow he left his backpack in the bathroom, he definitely deserves to be turtled.