The human body is full of all kinds of quirks. It makes confusing sounds seemingly out of nowhere. It jerks and jolts right before falling asleep or as the temperature changes. It yawns merely by reading the word “yawn” or seeing someone else do it.

These reflexes and involuntary responses reveal how little control people have over the way their bodies function. Understanding why some of these bodily reactions occur can give clues about things the body may need at a moment in time. For example, an athlete might get a cramp on the side of their ribs on a long jog when they’re fatigued. The body is telling them to reduce the intensity of their physical activity.

Other reactions offer insights into how humans have evolved to survive in many environments throughout history. The body shivers to keep warm in cold weather. Fingers and toes get wrinkly in aquatic environments to help people grip tools without slipping. Environmentally stimulated responses like these show just how dynamic and versatile the human body is for living in the Earth’s many climates.

But despite knowing what’s behind some of the quirks of the human body, biologists are still trying to figure out the explanations for other phenomena. Why do goosebumps cover people’s skin when they’re scared? Why do eyelids twitch randomly and uncontrollably? Why does the sun make some people sneeze? Sometimes it feels like there are more questions than answers for how the body works.

We compiled a list of 15 strange bodily reactions and looked at news articles, government reports, and scientific research to understand the biology behind these phenomena. Read on to learn the explanations for the peculiarities of the human body and get the answers to some reactions you may have always been curious about.

EXPLAINED: Wrinkled Fingers, Goosebumps and 13 Other Weird Body Quirks