From why joints click to what causes twins to be conjoined

Why do your joints click?

No one is really sure what causes joints to click, but there are several possible theories. Since the click is normally heard when the joint is moved or stretched, it is possible that the pressure of the synovial fluid in the joint cavity is reduced. This may create a gas bubble that makes a popping sound. Another explanation is that the sound is created by the separation of the joints’ surfaces, which release a vacuum seal in the joint.

Are women better multitaskers than men?

Probably. Studies show that, on average, women are better at dealing with several different stimuli at the same time than men. The critical words in that sentence are “on average”, which means that the average female score differs from the average male score, not that every woman is better than every man.

Evolutionary psychologists have suggested this may be because women bore the brunt of childcare in early human societies, so they had to respond to the demands of their children while remaining alert to danger and foraging for plant food. Men were responsible for hunting animals for meat and so had to be mentally focused. But it’s easy to devise theories like this; much harder to prove that they are correct.

Whose DNA was used to map the human genome?

The “genome” in the publicly funded human genome project does not come from one person. It was taken from a panel of 20 volunteers from a variety of ethnic and social backgrounds. Researchers collected tissue samples from the donors, and the names were protected, so neither donors nor scientists knew whose DNA was being sequenced.

The genome is different in every individual (apart from identical twins), but not very different. Around 99.99 per cent of our DNA is the same among humans. The map is an amalgam of the individual genomes. Since we all share the vast majority of our DNA, almost all of this map applies to everyone. But there will be differences. Only a few of the differences are in useful parts of the genome, and it is these that give rise to hair colour, blood groups and inherited diseases. Now that the genome has been mapped, geneticists will be able to locate more easily the differences in our genomes that make us individuals.

What causes twin babies to be born conjoined?

No one is really sure. It is probably the result of a mistake during development that would normally produce identical twins.

When a single fertilised egg divides very early on, while the embryo is no more than a ball of cells, it can lead to identical twins – though often one dies and the other embryo develops normally. Sometimes, however, both embryos develop, resulting in twins.

Conjoined twins result from the fertilised egg splitting incompletely, leaving some cells bridging the two developing embryos. That explains how the twins are physically joined. But it doesn’t explain the “why”.

Usually, when the fertilised cell fails to split completely, one or both embryos will be miscarried. Very rarely, the twins are born alive and conjoined. But, while conjoined twins are rare, the splitting of a fertilised egg is not. So rather than ask what makes the egg split (which scientists do not know), perhaps we should ask how the body knows when to miscarry and why this mechanism does not work in the cases of conjoined twins. Scientists do not know the answer to this question either.