From rubbing an injury to how many cells there are in the body

Why does rubbing an injury make it hurt less?

If you get hurt, your first response is generally to rub the area vigorously until the pain ceases. The rubbing activates receptors in the skin that act on endorphin-containing nerves. These release endorphins – the body’s natural painkillers – and help to stop the pain. Eventually, if the rubbing doesn’t work, you may have to resort to painkillers such as paracetamol and aspirin.

Why does the voice sound different when recorded?

Normally we hear our voices through the bones in our skull. The vibrations that we hear come through our own bones. When you record your voice and play it back, it sounds different because you hear the vibrations that have travelled through the air.

Are there sounds we can’t hear?

Yes. A young child’s ears can hear up to 25,000 hertz or more – the pitch (frequency) used by bats hunting for insects to eat. As we get older, we can only hear lower frequencies: by 13, most people can only hear sounds below 20,000Hz. Adults, especially those who have abused their hearing by working in noisy places, lose even more of the top frequencies, and some older people can only hear sounds below 5,000Hz. Luckily, most of the sounds in speech are in the range between 300Hz and 3,000Hz, so nearly everyone can still communicate. Human ears are most sensitive to sounds of about 3,000Hz. Maybe it’s no coincidence that this is the pitch of a smoke alarm or a baby crying.

Low-frequency sounds below about100 Hz feel more like a vibration than a note. The transformers in electricity substations vibrate at 100Hz; you’ll need to listen carefully because your ears don’t respond so well to such low frequencies, which sound more like a buzz than a note. A bee’s buzz, for example, is about 20Hz.

Why do we have five digits, not four?

The earliest four-footed animals – tetrapods – did not have five fingers or toes. Of the three we know well, Acanthostega had eight on each appendage, Ichthyostega seven and Tulerpeton six.

Subsequently, about 370 million years ago, tetrapods split into two major lineages – one that eventually gave rise to modern amphibians (and many extinct ones) and eventually gave rise to mammals, birds, dinosaurs and lizards. At this time, the number of toes on the foot stabilised at five, although some groups have subsequently lost some toes.

In the hand, however, something different happened. In the line going to amphibians, the number of fingers stabilised at five. In the line going to reptiles and mammals, fingers also stabilised at five. Scientists are still trying to deduce why.

Digit formation in mammals and birds is controlled by overlapping sets of genes, called hox genes. Older forms of life, such as fish, do not have hox genes. A genetic mutation must have occurred that spurred the development of fingers and toes in the line that led to tetrapods.

How many cells are there in the human body?

About 37.2 trillion. But although some cells, such as nerve cells, do not divide in the normal human adult, many others – such as blood cells – can and do divide in response to different challenges. Also, people vary in size and so must have different numbers of cells in their organs.