SpongeBob's tongue only appears alive in the episode "Fools in April."
It is a pink tongue that belongs to SpongeBob and has black eye pupils. It appears to be detachable from SpongeBob's mouth after long periods of the latter's laughter.
Role in episode
After laughing far too hard at the April Fool's prank he pulled on Tom, SpongeBob's tongue comes out of his mouth and starts laughing with him. SpongeBob tells his tongue that its shoes are untied, resulting in an even bigger laughfest than before and the former hugging the latter. He and his tongue then tell jokes via the Krusty Krab intercom.
SpongeBob's tongue also has sentient taste buds.