The kid was a bit sad, so he was blue.
Teacher asked him, "Why are you so blue?"
The kid replied, "I'm not sad."
Teacher said, "No, your face actually blue."
Alright, listen up, buttercup. This is a pun, see? The kid's "blue" because he's sad, but the teacher thinks he's LITERALLY blue, like his face is painted or something. You know, like you after you huff too much paint, genius.
Berbolate
Only 1 possible answer for that...
SMURF!!!!!!