Why foil makes your teeth hurt and other questions answered

Why does foil make your teeth hurt if, say, you eat chocolate and there’s some foil left on?

This is a well-known phenomenon when metallic objects come into contact with fillings. If the object is made out of a different type of metal than the filling, there will be galvanic action between the two.

Galvanic action occurs because different types of metal have a different likelihood of becoming ions. For atoms in the metal to become ions, there must be electrons available, usually from a solution such as water. So, if you have two metals, with different likelihoods of becoming ions, and water (or saliva) in between, electrons will move from one to the other. The movement of electrons is the same as saying that an electric current is flowing. So you get a tiny electric shock that sets off the nerve endings in your teeth, which hurts.

In an embryo, how do the cells begin to specialise? How do they decide, for example, to become brain cells?

This answer applies to mammals only. Other animals can have very different developmental strategies.

In the initial stages, the fertilised egg divides to form a ball of cells. Cell position in the ball gives the cue for the first differentiation step. Cells on the outer surface of the ball become one kind of tissue (which will contribute to the placenta only), and cells on the inside become another cell type, which will give rise to the baby that is born. It is believed that the number of cell contacts is the key feature: outside equals a few cell contacts, inside equals many.

Once you have two cell types, there is further scope for interaction. Some of the “outside” cells are in contact with “inside” cells, and some are not. This leads to another two different types of cell arising. And so it goes on in a cascade of increasing complexity. During that cascade, genes come into the picture. While every cell has the same set of genes, its position (outside/inside) determines which genes are read and used to make the proteins that the genes encode. Depending on which genes the cell uses, even more varying sets of genes are activated, and so on, leading to an astonishing number of different tissues such as brain cells and bones. All this from just one fertilised egg.

How can we tell if a sound is coming from in front or behind when we have only two ears?

The outside of the ear, the pinna, faces forward and catches sound waves. It also has several curves that help the ear determine from which direction the sound waves are coming. If the sound wave is coming from behind you, it will bounce off the pinna differently from sound waves coming from in front of or above you. The reflection of the sound wave off the pinna changes the sound wave pattern in a particular way, which depends on the direction it has come from. The brain is able to tell the difference between these patterns and so can tell which direction the sound is coming from.

The human ear is less sensitive to sounds from behind the head because of the immovable, forward-facing pinna. Other mammals, such as cats, can move their ears to point towards the source of the noise.

Does the body really burn fat from the immediate vicinity of an exercised area? In other words, if I do loads of sit-ups, am I not just toning the stomach muscles?

The body doesn’t really burn fat from the immediate vicinity of an exercised area if you are doing resistance work, eg, sit-ups, weights or toning exercises. All you are doing is strengthening and toning the muscle you are exercising beneath the fat. To burn fat, you need to do cardiovascular exercise: running, cycling, swimming, etc. The optimum heart rate for burning fat is at 60-65 per cent of your maximum heart rate, which is roughly 220 minus your age equals beats per minute. So if you were 20, your maximum heart rate would be 220-20 = 200bpm, and 60 per cent of 200 = 120bpm, and 65 per cent of 200 = 130bpm. So the optimum fat-burning exercise for a 20-year-old is any aerobic exercise providing their heart rate is between 120 and 130bpm. Where you burn fat from is thought to be determined genetically, but basically, men normally store fat on their stomach and women on their hips. So exercise is most likely to burn fat from those areas.