The Adventures of a Brownie by Dinah Mulock

WHICH was a place where he did not often go, for he preferred being warm and snug in the house. But when he felt himself ill-used, he would wander anywhere, in order to play tricks upon those whom he thought had done him harm; for, being only a Brownie, and not a man, he did not understand that the best way to revenge yourself upon your enemies is either to let them alone or to pay them back good for evil -- it disappoints them so much, and makes them so exceedingly ashamed of themselves.

One day Brownie overheard the Gardener advising the Cook to put sour milk into his bowl at night, instead of sweet.