A tannin is a natural chemical found in plants that creates a bitter, dry feeling in your mouth. If you've ever bitten into an unripe banana or persimmon and felt like your tongue was turning into sandpaper, you've experienced tannins at work.

Plants produce tannins to protect themselves from being eaten. The bitter taste and mouth-puckering sensation discourage animals from munching on leaves, bark, or unripe fruit. Oak trees are especially rich in tannins, which is why their acorns taste so terrible to us but can be eaten by squirrels and deer, whose digestive systems handle tannins better than ours.

Tannins have been incredibly useful throughout history. For thousands of years, people used tannins from oak bark to turn animal hides into leather through a process called tanning. The tannins bind to proteins in the hide and preserve it, preventing it from rotting and giving leather its durability.

You encounter tannins every day in tea, which gets its slightly bitter taste from tannins in the tea leaves. Red wine, dark chocolate, and walnuts also contain tannins. When tea steeps too long and tastes overly bitter and astringent, that's because the tannins have become too concentrated.