Some things your body does without asking you. You cannot decide when a sneeze will happen, because a sneeze is a reflex, which means your body does it on its own. What about hiccups and yawns? Let's look at all three.

A hiccup begins with the diaphragm, a muscle under your lungs that helps you breathe. In a hiccup, the diaphragm jerks, or spasms, and you cannot stop it. Air rushes in fast, and then the vocal cords snap shut, which makes the "hic" sound. Eating or drinking too much or too fast can start hiccups, and so can fizzy drinks.

People try many home tricks for hiccups, such as holding their breath, sipping ice-cold water or swallowing a little sugar. 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 science article for children reports a big study that found none of them actually work.

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. The sneeze helps clear dust and dirt out of your nose.

A sneeze can send up to 40,000 tiny droplets out of the nose and mouth. Sneezes and coughs can spread germs, such as the germs that cause a cold. Washing your hands with soap and water after you sneeze 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. 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.

Why do we yawn? Scientists used to think yawns bring in more oxygen, but studies did not support that idea. One newer 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.

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. Dogs can catch a yawn from a person, too.

So what is a yawn for? Nobody knows for certain. The next time you yawn, remember that scientists have more than one idea about why.

Words to know

• reflex: automatic action
• diaphragm: breathing muscle
• theory: possible explanation
• oxygen: gas we breathe