Ace gives the information you need on donuts.
Plain donuts are also called "not donuts" or "ring-shaped bread."
Plain donuts were invented for parents who don't love their children. They are also sometimes put out as bait for poisoning rats, though they have a 75% failure rate. Rats don't like them either. Sometimes a poisoned plain donut will be found intact, with a dead rat next to it -- rats will lick the poison off the plain donut while avoiding the plain donut itself.