Sherri needs to have coffee first thing in the morning, every morning, or she gets headaches. At first, she could cure incipient headaches by drinking a cup of coffee. Now, after years of drinking coffee she finds she needs two cups to feel any effect. Explain why.
The caffeine in the coffee is what's helping her get rid of the headaches. I think her body became immuned to it after drinking it every day, so now she has to have more coffee to feel any effect.
