When change is a brewin', remember: to ferment is to cause a chemical change to food or drink, like turning grapes into wine, but to foment is to stir up trouble, like turning a group of people into an angry mob.

Fermentation is a magical alchemy through which bacteria, yeast, and time turn barley into beer, cabbage into kimchi or sauerkraut, and milk into kefir. To ferment is to effect that change. Since ferment describes a process, it's often used in the form of fermented or fermenting. Fermented grapes = wine.

To ferment can also mean to stir up. When you ferment something, you agitate it, you work it up, and then it changes. You can ferment people, too! An inspiring leader might ferment a revolution! See the word in action:

Thus campaigns demonstrate that creationists threaten the creative ferment that produces social improvement. (Washington Post)

Miso is a fermented soy product rich in probiotic cultures, which are essential for immune health. (US News)

Elephants in southern Africa often binge on the naturally fermenting fruit of the marula tree and lumber unsteadily away afterward. (Wall Street Journal)

Foment is similar to ferment in the sense of stirring up people, but it's always bad. To foment is to cause trouble.

The word's Latin roots mean to apply a warm compress, and it can still be used that way, but usually it's more like slapping someone with a towel dipped in hot water — start fighting! Here are some examples from the news:

We want you to model interpersonal reconciliation rather than foment personal conflict. (Time)

They said they would use their strategic reach and inspirational force to foment domestic attacks in the United States, and they did in San Bernardino. (US News)

It can even foment terrorism as people give up hope in good and honest government. (BBC)

So, use ferment when discussing drunk elephants and happy sports fans, and foment for angry rebellions. It's what bad guys do, and not to food.