Ask them to do a trick and they'll give it a try. For a reward - say, sausage - they'll happily keep at it.
But if one dog gets no reward, and then sees another get sausage for doing the same trick, just try to get the first one to do it again. Indeed, the dog may even turn away and refuse to look at you.
Dogs, like people and monkeys, seem to have a sense of fairness.
"Animals react to inequity," said Friederike Range of the University of Vienna, Austria, who led a team of researchers testing animals at the school's Clever Dog Lab. "To avoid stress, we should try to avoid treating them differently."