A clam is a type of shellfish that lives buried in sand or mud at the bottom of oceans, bays, and rivers. Clams have two hard shells hinged together that protect their soft bodies inside. When danger approaches, a clam can snap its shells tightly shut, which is why we say someone who refuses to talk has clammed up or is being as silent as a clam.

Clams use a muscular foot to dig themselves into the sand, where they filter tiny food particles from the water. Some clams are tiny, while others, like the giant clam, can grow several feet across. People have harvested clams for food for thousands of years. You might eat clams in chowder, steamed with butter, or fried as clam strips.

The phrase happy as a clam comes from the longer saying “happy as a clam at high tide.” At high tide, clams are safely covered by water and hidden from the birds and people who might dig them up, so they're about as content as a clam can be. When someone describes you as happy as a clam, they mean you're perfectly satisfied and comfortable with how things are going.