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Q. Why does water bubble when it boils?

A. As water is heated, its molecules become more excited as they gain extra heat energy and move around a lot more. At a certain temperature, some molecules have enough energy to change into a gas and become water vapour. This change can take place within the body of water. Since the water vapour is less dense than the liquid, it rises in the form of expanding bubbles until it reaches the surface of the water and escapes.

Q. How does amount of body fat relate to fitness?

A. It depends on the sport. Athletes such as runners and high-jumpers try to minimise body fat, which they see as dead weight. It requires energy to move it, but does not increase the body's power output. In aerobic sports such as cycling where weight is not especially important, athletes still try to reduce body fat. This is because performance in aerobic sports is often limited by the maximal rate of oxygen uptake. Fat tissue is actively metabolising, using oxygen and requiring blood circulation. If fat is lost, the oxygen and blood flow can be redirected to muscles, which give a higher power output.

Swimmers tend to have higher fat levels than most athletes. This is because fat is less dense than muscle and increases buoyancy. For sumo wrestlers, the advantages of a huge percentage of body fat and weight are quite obvious.

Q. What causes winds?

A. The sun heats air in some regions more than others, causing it to expand and become less dense. It rises and creates a lower pressure in the area below. Air rushes in from the nearest high pressure area to fill the gap, and places between the two pressure zones experience wind.

Q. Why are metals better thermal conductors than glass and wood?

A. A defining property of a metal is its structure. Metals are made of a lattice of positive ions (atoms with one or two electrons missing) in a sea of the missing electrons, which are very mobile and free to conduct electricity and heat. Glass has a structure in which all electrons are firmly attached to atoms, meaning they do not conduct electricity.

Q. Why does salt placed in ice buckets cool wine quicker?

A. Salt lowers the freezing point of ice, causing the ice to melt. But to melt, the ice needs energy. It takes this from the wine, thereby cooling it.

n These questions and answers are provided by Science Line. You can use its Dial-A-Scientist service on 0345 600444.