Chew the cud is an expression that refers to the process of rumination, which is the act of chewing food again after it has been swallowed. This behavior is most commonly seen in animals such as cows, goats, and sheep, who are able to extract more nutrients from their food by regurgitating and re-chewing it. Ruminants have a four-chambered stomach which allows them to break down their food more efficiently. The phrase 'chew the cud' is used to describe any kind of thoughtful consideration or contemplation.