What does Jem think is hiding under Scout's bed?

From Scout's description, Jem believes a snake is hiding under Scout's bed. In reality, Dill is hiding under the bed. He's run away from home.

“Are you bein‘ funny?” Jem opened the door. He was in his pajama bottoms. I noticed not without satisfaction that the mark of my knuckles was still on his mouth. When he saw I meant what I said, he said, “If you think I’m gonna put my face down to a snake you’ve got another think comin’. Hold on a minute.”

Suddenly a filthy brown package shot from under the bed. Jem raised the broom and missed Dill’s head by an inch when it appeared.