Snake in copybook

One boy started to draw in his copybook at the lesson instead of writing from the blackboard. He drew some kind of stick which reminded a snake a lot. As soon as he started to draw a scratch he noticed that the stick which turned into a snake has eaten this scratch. The boy was scared and took his pen off the copybook. The snake kept on moving. He tried to draw another scratch, but it was eaten by the snake as well. And that happened again and again and again...