Stand outside the school cafeteria passing out flyers with nutritional details on school food, and you may foment a revolution — foment means stirring up something undesirable, such as trouble.
You would never say, "Hooray, we fomented a revolution." Instead you'd say, "Those good for nothing scalawags fomented the rebellion." Don't confuse foment and ferment. Ferment can mean "to stir up" in a good way — a football game can ferment excitement in a town, or foment trouble through traffic tie-ups and litter.