Have you ever sucked on a lemon and felt your face scrunch up? Foods that are very sour contain a lot of acid and can make you pucker—wrinkle your face, squint your eyes, and press your lips together. When things like lemons, vinegar, and unripened fruit touch your tongue, your brain gets a signal that you’re eating something sour. It could be your body's way of saying "watch out!"
Your tongue has thousands of little bumps with tiny sensors called taste buds. Taste buds let you know when something is sweet, salty, sour, bitter, or savory. (Savory is also called umami. Say: ooo-MOM-eee.) Each taste bud has dozens of taste cells that have little sprouts on them that look like hair that can only be seen with a microscope. When foods dissolved in your saliva touch them, they tell the brain about the flavor of what you are eating. When they come in contact with very sour foods, your face might pucker up because the taste is strong and acidic.
Puckering when you taste something sour is often involuntary (in-VAWL-uhn-ter-ee). That means you do it without trying. It may happen because we have an instinct not to eat things that are dangerous. Of course, not all sour foods are bad for us. But some sour foods can make us sick—spoiled milk or fruit that is not ripe, for example. Reacting with a wrinkled-up face may be our body’s way of trying to warn ourselves and others to stay away from foods that might hurt us.
For further reading, check out “Why Are Lemons Sour?” over at Wonderopolis.