Live Science: Why do people scrunch up their faces after tasting something sour?

Here’s an activity: Get a lemon, peel it and keep a straight face while you eat the whole thing.

Could you do it? No, probably not. How is it that a fist-size fruit has the power to make you move your muscles against your will?

Scientists don’t know for sure, but there’s a good chance that the answer involves three things: protons, vitamin C and the tropical-fruit buffet our ancestors were enjoying back when they lived in trees.

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