Kevlar is able to stop a bullet due to its molecular structure. It is a light, polyarylamide plastic fabric, which has a high tensile strength. this means it takes a huge amount of energy to make its fibres stretch even a little. Each Kevlar molecule looks like a long twisting coil. During polymerisation these coils of molecules become tangled, causing it to be hard to stretch.

Inside a bulletproof vest are many strips and layers of Kevlar. When a bullet hits the vest, it tries to force it through the layers, but to do this it must push the fibres apart. The fibres are woven and resist this very effectively. The movement is translated into a stretching force on the fibres. Some will break, but most will absorb the energy of the bullet by stretching a small amount.

Nathaniel Marten, Science Museum

Ant strength is due to their small size. An ant worker can lift an object weighing five to ten times its weight and can drag a 20-50 times heavier object, whereas humans can rarely lift their own weight. However, ant muscles are no stronger than human muscles (in terms of force per cm2), but the small size of ants (one to five milligrams) gives them an advantage on how much muscle force they can produce. As the size of an organism increases, its body mass increases at a much greater rate than the cross-sectional area of muscles, so that the muscles of larger organisms have proportionately more mass to lift. A small size means ants have proportionately more muscle (in terms of cross-sectional area) that they can use to lift heavy objects. If we were as small as an ant, we could do the same.

Answered by Dmitri Logunov.

The brain itself may not have any pain-sensitive nerve receptors, but that doesn’t mean the inner head can’t experience pain… as anyone who’s had a headache will know. The most common form of headache is the tension headache. When the muscles in your body stay semi-contracted for a period of time – for example, when we feel stressed and can’t seem to relax – this is known as muscle tension. Such tension in the meninges (the membranes that help protect the brain), or the face, neck and scalp muscles activate the body’s pain receptors, sending impulses to the brain’s sensory cortex and signalling pain and causing a headache.

More frequent in women than men, the primary tension-type headache manifests itself as a dull ache across both sides of the head. Secondary headaches, meanwhile, can be caused by an underlying health condition such as meningitis, a blow to the head or other sinus-related ailments.

Emotions

While great apes such as gorillas, chimps and orangutans use facial expressions to show their feelings, human beings are the only animals to cry as a result of their emotions.

Sneeze fast!

A sneeze is typically expelled at around 161km/h (100mph). Sneezing helps protect the body by keeping the nose free of bacteria and viruses.

Red blood cells

Red blood cells – also known as erythrocytes – live on average for 120 days. There are approximately 2.5 trillion red blood cells in your body at any given moment.

Hard worker

The heart beats on average 100,000 times per day. Of course this will vary greatly depending on your level of activity and your environmental conditions.

Liquid

Humans are made up of 70 per cent water, which is essential for body growth and repair. The NHS suggests drinking 1.2 litres of water a day to avoid growing dehydrated.

When you feel your own pulse, you’re feeling the direct transmission of your heartbeat down an artery. You can feel a pulse where you can compress an artery against a bone, for example the radial artery at the wrist.

The carotid artery can be felt against the vertebral body but beware – a) press too hard and you can faint, b) press both at the same time and you’ll cut off the blood to your brain and, as a protective mechanism, you’ll definitely faint!

HIW

Asked by Harry Blythe

Glowworms, a disparate nomenclature for a disparate collection of insects (none are actual worms) glow via a variety of different methods, but all emit light via bioluminescence. Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in the form of light. For example, in phengodes glow is produced and emitted from giant solitary cells within the abdomen, while in arachnocampa it is emitted through modified excretory organs.

In addition, glowworm bioluminescent organs vary widely in size, location and structure, indicating that the different species’ glowing abilities evolved independently of one another. This is supported by the reasons for producing the glow, which range from attracting a mate through to warding off predators and luring prey into sticky webs for consumption, the latter characteristic of the platyura species.

Fainting, or ‘syncope’, is a temporary loss of consciousness due to a lack of oxygen to the brain. It is usually preceded by dizziness, nausea, sweating and blurred vision.

The most common cause is overstimulation of the body’s vagus nerve. Possible triggers of this include intense stress and pain, standing up for long periods or exposure to something unpleasant. Severe coughing, exercise and even urinating can sometimes produce a similar response. Overstimulation of the vagus nerve results in dilation of the body’s blood vessels and a reduction of the heart rate. These two changes together mean that the body struggles to pump blood up to the brain against gravity. A lack of blood flow to the brain means there is not enough oxygen for it to function properly and a fainting episode occurs. However, shortly after fainting, the blood flow to the brain is restored (usually from simply lying down as a result of the fainting) and the brain functions normally again.

Heavy bleeding, diabetes, drugs and low blood pressure can similarly reduce blood flow to the brain causing a lack of oxygen and fainting.

Josh Moore, Science Museum

When you see flies sitting on the windowsill rubbing their legs together, they are cleaning themselves. Flies rely on their compound eyes, antennae and the bristles on their bodies and legs to sense the world around them. They have to keep these sense organs clean so they can fly with precision to find food and mates and avoid predators.

Flies spend a long time carefully cleaning their whole bodies; if you watch them closely you will see that they don’t just rub their legs together – they run their legs over their bodies, heads and wings too. Particles of dirt from the rest of the body are picked up by its legs as it cleans, and then the dirt is rubbed down the legs and away.

Hannah Cornish, Angela Marmont Centre, National History Museum, London

This may take many years to be answered, if it’s ever done. Our brain processes information in a way that is very different from regular computers. We’re only just starting to understand how it stores memories and how these memories are linked together. Many people tried to make estimates about how it would compare to a computer, and common estimates range between one and ten terabytes (1,000 gigabytes). Others believe it is far greater at about two petabytes (a million gigabytes). Some think it’s actually much smaller, and our brain is just much more efficient than computers at storing information.

José Monteiro, Science Museum

Asked by Lady Lightning on the How It Works forum

That ‘lump’ is actually not a lump at all but a counter-reaction to the body’s automatic nervous system. When humans are exposed to stressful situations – ie, situations that would cause them to cry or get angry – the body, due to the genetic evolutionary ‘fight or flight’ nature of humans, automatically increases blood flow to vital organs and muscles.

Unfortunately, one of the ways the body achieves this is by opening the glottis (the vocal folds in the throat that humans use to generate vibrational noise) in order to allow your lungs to receive more inhaled oxygen than normal. By doing this, while increasing available oxygen which can be beneficial in stressful moments, it causes any human who wishes to swallow to fight against their body’s automatic nervous system for control of the glottis’ positioning, causing that distinctive sore pain in the throat.