A tail is the part that sticks out from the back end of an animal's body. Dogs wag their tails when they're happy. Cats use their tails for balance when climbing or leaping. A monkey's tail works almost like an extra hand, helping it swing through trees. Fish swish their tails side to side to push themselves through water. Birds have tail feathers that help them steer while flying, working like a ship's rudder.

Different animals use their tails in surprisingly clever ways. A beaver slaps its flat tail on the water to warn other beavers of danger. A rattlesnake shakes the rattle on its tail as a warning. Horses swat flies with their tails. Some lizards can even break off their tails to escape predators, then grow new ones later.

The word also describes something that follows behind, like the tail of a kite streaming through the sky or the tail of a comet stretching across space. When you're at the tail end of a line, you're at the very back. And if you tail someone, you're following them, often secretly, the way a detective might tail a suspect through city streets.