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For many people, the aroma of freshly brewed coffee is the start of a great day. But caffeine can cause headaches and jitters in others. That’s why many people reach for a decaffeinated cup instead.

I’m a chemistry professor who has taught lectures on why chemicals dissolve in some liquids but not in others. The processes of decaffeination offer great real-life examples of these chemistry concepts. Even the best decaffeination method, however, does not remove all of the caffeine—about 7 milligrams of caffeine usually remain in an 8-ounce cup.

Producers decaffeinating their coffee want to remove the caffeine while retaining all—or at least most—of the other chemical aroma and flavor compounds. Decaffeination has a rich history, and now almost all coffee producers use one of three common methods.