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Q. Is it true that brain cells start to die off after we're born?

A. Brain cell death does occur as soon as we're born, although these cells are replaced by new ones. But after the age of 20, dying cells are not replaced and the brain starts to lose about a gram of its weight per year. This must mean that some of the 100 billion nerve cells duplicate tasks, otherwise we wouldn't function very well by the time we reached 70.

Q. What is the fastest thing on earth?

A. It all depends on what you mean by thing. Here are some very fast things you can find on this planet, so take your pick.

1. Light. Nothing can move faster than light. It travels at 186,000mph.

2. A spacecraft. Both the Russian space station Mir and the US Space Shuttle orbit the earth at about 25,000mph. But, then, they're not actually on the earth.

3. Birds. It is very difficult to measure how fast a bird flies. Birds swoop all over the sky rather than fly in straight lines. But the spine- tailed swift has been measured by Russian scientists at 106mph. The fastest bird in Britain is the peregrine falcon, with a dive speed of 82mph.

4. The cheetah. It is thought to be the fastest animal on land. Its fastest recorded speed is 51mph, though many scientists think it can go as fast as 60mph.

Q. Why do we itch?

A. Itching is an early warning that the body has come into contact with a noxious substance. We itch because we release a substance called histamine from "mast" cells in the skin's connective tissue. Histamine binds to special receptors on local nerve endings, causing the sensation of itching. This reflex can be stimulated by various irritants. People with allergies overproduce histamine in response to a substance that may not affect other people. A good example is pollen and hay fever.

Q. Why doesn't the stomach dissolve itself?

A. Mucus produced by goblet cells in the stomach wall forms a protective layer. This stops the pepsin and hydrochloric acid from breaking through the stomach tissue. If this protection is not effective - for example, if there's an uncovered portion of lining, or the covering is thin - the gastric juices can attack the mucosa and an ulcer can form.

Q. Why do people get bags under their eyes?

A. Nobody seems to know - not even the Quaker Labs in the US, which do research into fluid balance within the body. One idea is that the loose tissue under the eye swells as a result of a drop in blood pressure.

Q. What is the lowest intensity of light that the human eye can detect?

A. A single photon of light is enough to cause a rod in the human eye to fire. Rods are one of the eye's two types of light-detecting cell. The other type, the cone, is used to detect colour and needs to be exposed to several photons before it is activated. This is why rods are more important than cones for vision in low light conditions. Cones don't work well enough to allow us to see colours at night.

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