There are some things about our bodies that just don’t make sense, but many of these traits aren’t as bizarre as they may seem. In fact, many have an evolutionary tale or sound scientific explanation behind them. Here are some quick answers to 10 weird puzzles of human biology, kicking off with…

Why do men have nipples?

No matter how macho the end result, all male embryos start off as female and nipples are a souvenir of this androgynous origin. Every foetus contains the genetic information to be either male or female, but if there is a Y chromosome present in the embryo then after 60 days the hormone testosterone starts firing and changes the genetic activity of cells in the genitals and brain.

When we’re tired, why do we get bags under our eyes?

Dark rings are caused by thin skin around the sockets

Blood doesn’t circulate around your body as efficiently when you’re asleep, so excess water can pool under the eyes, making them puffy. Fatigue, nutrition, age and genes also cause bags. Dark rings are more likely the result of blood vessels showing through the thin layer of skin around your eyes.

What is the point of tonsils?

The tonsils are collections of lymphatic tissues which help to fight off bacteria and viruses from the upper respiratory tract. However, they themselves can sometimes become infected – leading to tonsillitis. The ones you can see at the back of your throat are just part of the ring of tonsils. You won’t miss them if they’re taken out for recurrent infections though, as the rest of your immune system will compensate.

Why are we ticklish?

Being ticklish could be an evolutionary response to predators

Light touches, by feathers, spiders, insects or other humans, can stimulate fine nerve-endings in the skin. This stimulus sends impulses to the somatosensory cortex, an area of the brain that processes input from the various systems in the body that are sensitive to touch. Certain areas are more ticklish – such as the feet – which may indicate that it is a defence mechanism against unexpected predators. It is the unexpected nature of this stimulus that means you can be tickled. Although you can give yourself goosebumps through light tickling, you can’t make yourself laugh.

What makes us left-handed?

One side of the brain is typically dominant over the other. Since each hemisphere of the brain controls the opposite side (ie the left controls the right side of your body), left-handed people have a more dominant right brain hemisphere. Occasionally you’ll find an ambidextrous person, where hemispheres are co-dominant, and these people are equally capable with both right and left hands.

Why do bruises go purple or yellow?

Bruises are caused by damaged capillaries under the skin

A bruise forms when damaged capillaries under the skin leak and allow blood to settle in the surrounding tissues. The haemoglobin in red blood cells is broken down, and these by-products give a dark yellow, brown or purple discolouration depending on the volume of blood and colour of the overlying skin. Despite popular belief, you cannot age a bruise – different people’s bruises change colour at different rates.

Why does cutting onions make us cry?

When you cut an onion they expel an irritant gas called syn-propanethial-S-oxide. When the gas reaches a person’s eyes it activates sensory neurons around the eyes and creates a stinging sensation which, in turn, causes tears. The human body uses tears to cleanse the eye of debris and dilute any irritants. You can reduce the effects of this volatile gas by submerging the onion in water before or during the chopping.

Do eyeballs grow like the rest of the body?

Your eyeball is almost fully grown when you are born

Only by a very small amount – this is why babies appear so beautiful, as their eyes are slightly out of proportion and so appear bigger.

Why do we burp?

A burp is a natural release of gas from the stomach. This gas has either been swallowed or is the result of something you’ve ingested – such as a fizzy drink. This excess gas escapes the stomach, traveling up through the oesophagus and out through your mouth.

Why can some people roll their tongues but others can’t?

It might be due to a mix of learned behaviour and genetics

Although we’re often taught in school that tongue rolling is due to genes, the truth is likely to be more complex. There is likely to be an overlap of genetic factors and environmental influence. Studies on families and twins have shown that it cannot be a case of simple genetic inheritance. Ask around – the fact that some people can learn to do it suggests that in at least some people it’s environmental – a learned behaviour – rather than genetic (inborn).