If someone says you're crying crocodile tears tears, they don't mean you're sad. They mean you're faking it. You're crying, or pretending to cry, when you're really not sad at all. In fact, you may be quite gleeful. This saying arose out of the belief that crocodiles cry as they devour their prey. Since crocs enjoy their meals, they must therefore be feigning sadness about taking another creature's life.
Crocodiles do shed tears (as do alligators), meaning they have lachrymal glands that produce tears, which lubricate their eyes. And yes, they do sometimes shed these tears while they eat. Researchers at the University of Florida believe the animals get so excited when they're chowing down that air gets blown up through their sinuses and forces tears back up into the eye. Yet while there's no sadness behind their tears, there's no emotion, fake or otherwise, behind them either. Their tears are merely a physiological response, like sweating when you're nervous.