Why do crocodiles cry when they eat their prey?

They do not. While crocodiles do possess lacrimal glands and can produce tears, they do not (or are not known to) cry; the glands simply provide moisture and lubrication for the eye. The idea of crocodiles crying (or 'weeping') while they eat probably comes from the same ancient anecdote that states crocodiles have no tongue (which is also false).