Hiccups are a reflex: the diaphragm contracts suddenly, which causes a sudden intake of breath that is cut off when the glottis closes. Nobody knows for sure why we hiccup, and hiccups have no obvious purpose. One idea is that they did something useful in our distant relatives and never left us. One theory says our hiccup reflex may come from our amphibian ancestors: tadpoles have a hiccup reflex that helps keep their lungs safe while they switch from breathing underwater. Unborn babies hiccup too.

A sneeze is a sudden burst of air that rushes out of your nose and mouth, and it is a reflex, which means your body does it on its own. Some people sneeze when they step from a dim place into bright light, which is called the photic sneeze reflex. As of November 2026, estimates of how many people sneeze in bright light vary: about 25% in a 2025 research paper, one in three at Cleveland Clinic, and 10 to 30 percent in a PBS report. Scientists are not sure how light makes people sneeze. The most popular theory is that signals cross between the optic nerve and the trigeminal nerve, which senses facial feelings like an itchy nose.

A yawn is a deep breath in with the mouth wide open, often with the eyes closing and a stretch of the body. Nobody knows for certain why we yawn. You may have heard that a yawn brings in more oxygen. Scientists used to think so, but studies did not support that idea. A fetus gets its oxygen from the blood through the umbilical cord, yet it still yawns, which is one reason researchers doubt the oxygen idea. Three main theories try to explain yawning: it wakes up the brain, it cools the brain, or it sends a message to others. The arousal theory says yawning helps wake the brain up, because yawns are most common when people are drowsy or bored.

One idea, the brain-cooling theory, says a yawn pulls in cool air that cools the blood going to the brain. It is a theory, not a settled answer. In a 2007 study led by psychology professor Andrew Gallup, people who were warmed yawned more at pictures of yawning, and people in a cooler place or with cold packs on their foreheads yawned less. In 2014, researchers from the University of Vienna measured contagious yawning outdoors in Vienna in summer and winter and compared it with an earlier Arizona study: people in Vienna yawned more in summer, and people in Arizona yawned more in winter.

The communication theory says a yawn may tell the people around you that you are bored or stressed, or that the place is not right, which could be why yawns spread. Yawns are catching: when you see someone yawn, you often yawn too. We are much more likely to catch a yawn from a close loved one than from a stranger. Researchers think contagious yawning is related to empathy, our ability to understand and share the feelings of another. The idea is still being tested.

Words to know

• reflex: automatic body action
• theory: possible explanation
• oxygen: breathing gas
• empathy: sharing feelings
• photic: about light