Ferret out is a common expression that means "to hunt" or "search for."  This expression originated centuries ago when people used tame ferrets to drive rabbits and rodents out of hiding places. 

Ferrets are not related to rodents, such as rats and mice.  Instead, they are mustelids, and are closely related to weasels, minks, otters, badgers and skunks.  They belong to a scientific family called Mustelidae.

Ferrets were domesticated before the cat, probably by the Egyptians.

In ancient Egypt, people kept tame ferrets as pets and rodent hunters.

The Greeks and Romans also used ferrets to get rid of rats and mice and to catch rabbits.

Ferrets were sometimes used in industry -- such as in manufacturing airplanes -- and in construction to carry wires and lines through narrow pipes and tubes.

An adult male ferret, called a hob, may grow as long as 24 inches (including the tail) and weigh as much as 5 pounds.

An adult female ferret, called a jill, may grow as long as 18 inches and weigh up to 3 pounds.

Ferrets like to take small objects or food and hide them.

Black-footed ferrets were nearly extinct, but in 1986, 18 of them were captured and by 1991 there was more than 200.  Once again, black-footed ferrets live on the North American plains.