A "tail" is a flexible, elongated structure that extends from the rear of many animals, including mammals, reptiles, and birds. It serves various functions, such as balance, communication, and locomotion. For example, a cat uses its tail for balance while climbing, while a dog may wag its tail to express excitement. In addition to animals, the term "tail" can also refer to the rear part of an object, such as the tail of an airplane, which helps with stability during flight. In some contexts, "tail" can describe the end part of a comet, known as the comet tail, which forms as it approaches the sun.