Why do you do things you did not plan? A sneeze is a reflex, which means your body does it on its own: you cannot decide when it happens. Hiccups and yawns are worth a look too. Scientists know quite a bit about each one, and they are unsure about some parts.

In a hiccup, the diaphragm, a muscle under your lungs that helps you breathe, jerks, or spasms, and you cannot stop it. Air rushes in fast, and then the vocal cords snap shut, which makes the "hic" sound. Hiccups most often come from a stretched, full stomach: eating or drinking too much or too fast.

Most hiccups stop within minutes. If hiccups last longer than 48 hours, a grown-up should take the person to a doctor. People try home tricks, such as holding their breath, sipping ice-cold water or swallowing a little sugar, but these are things people try, not proven cures. The NHS says there is no evidence that home hiccup tricks work for everyone, though many people find them helpful.

A sneeze starts when something irritates the lining of the nose or throat, and sensory nerves send the alarm. Dust, pollen, spicy food and strong feelings can set off a sneeze. It helps clear dust and dirt out of your nose. Some people also sneeze when they step from a dim place into bright light, which is called the photic sneeze reflex.

A sneeze can send up to 40,000 tiny droplets out of the nose and mouth, and sneezes and coughs can spread germs, such as the germs that cause a cold. When you sneeze, cover your nose and mouth with a tissue, or sneeze into your elbow if you have no tissue. Then wash your hands with soap and water, which helps stop germs from spreading.

A yawn is a deep breath in with the mouth wide open, often with the eyes closing and a stretch of the body. Adults yawn about eight to nine times a day on average. Babies yawn before they are born, and cats, dogs, birds and fish yawn too.

Scientists used to think yawns bring in more oxygen, but studies did not support that 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. People tend to yawn most when they have just woken up or when they are tired. People also tend to yawn when they are nervous or before an important event.

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. 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. Yawning can make a person yawn even if they only hear or think about a yawn, and dogs can catch a yawn from a person, too. Nobody knows for certain why we yawn.

Words to know

• reflex: automatic action
• diaphragm: breathing muscle
• arousal: waking up
• communication: sending messages