In 1997, a 7-month-old sheep named Dolly became a celebrity. Dr. Ian Wilmut, a Scottish scientist, announced to the world that he had created her using a procedure called cloning. Cloning is a method that scientists use to produce a genetic copy of another individual. In other words, Dolly is a clone of her mother.

Well, actually, Dolly had three mothers. One mother gave Dolly her DNA, one mother supplied an egg, and the third mother, her surrogate mother, gave birth to her.

Normally, an animal gets half of its DNA from its mother and half from its father. Dolly is an identical twin of the mother who gave her her DNA. But Dolly is six years younger.