The Wolf, the Goat and the Kid

As an old Goat was going forth to pasture, she carefully latched her door, and bid her kid not to open it to any one who could not give this pass-word: "Beware of the Wolf and all his race." A Wolf happened to be passing, and overheard what the old Goat said. When she was gone, he went to the door, and, knocking, said: "Beware of the Wolf and all his race." But the Kid, peeping through a crack, said: "Show me a white paw and I will open the door." As the Wolf could not do this, he had to depart, no better than he came.

Two sureties are better than one.