The 'ceiling' is the top of the room. In the picture you can see the boy is almost touching the ceiling because the monster is so big!
How do you know if there is a monster under your bed?
Because your head is almost touching the ceiling!
Do you like this joke?
Hahahahahahahahahahahahahahahahahahahahaha!!!!!!!!!!!! This joke makes me laugh a lot. That monster is huge! I am afraid of monsters.