The joke is a bit of wordplay that relies on a twist of expectation. In the context of the comment, there's an implication that the crocodile is laughing so hard that it can't close its mouth, which is not something you would expect a crocodile to do—crocodiles typically aren't portrayed as humorous creatures.
The humor in the comment likely comes from the absurdity of the idea that a crocodile would be laughing at the situation, causing it to keep its mouth open. In reality, if a crocodile were to bite, it would probably be more likely to do so out of instinct or aggression, not amusement.
The man pulling his hand out of the crocodile's mouth reflects a natural instinct for self-preservation, as he was surprised and scared by the potential danger. The comment suggests that instead of being scared, the crocodile found the whole situation amusing, which adds a humorous and unexpected element.
So, the joke's humor is in the unexpected idea of a crocodile laughing, making it a silly and absurd image. If the comment didn’t provide enough context or was a bit confusing, that’s understandable!