The laugh box is a fictional body part that only appears in the episode "Funny Pants."
The laugh box is a larynx-like body part. It is presumed to be the body part that allows people to be able to laugh.
Role in episode
Fearing the loss of his laugh box after being told not to laugh too much by Squidward, SpongeBob goes 24 hours without laughing only to find that he is unable to laugh when he wakes up. After failing to laugh even after consulting his friends, SpongeBob spends the night crying over losing his laugh box.
Eventually Squidward cannot stand it and tells SpongeBob that he was lying to him. They both laugh for a while until SpongeBob leaves in anger and Squidward soon is taken to the hospital overusing his laugh box after not laughing for too long. SpongeBob volunteers to donate part of his laugh box at the hospital and the doctors replace Squidward's with SpongeBob's. At the end of the episode, it is revealed that Squidward is now stuck with SpongeBob's laugh.