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Questions and answers provided by Science Line's Dial-a-Scientist on 0345 600444

Q Are green potatoes poisonous?

A Potatoes go green when they are exposed to light. This green colour is chlorophyll, one of the pigments plants use to trap the energy in sunlight. Chlorophyll itself isn't poisonous - but solanine, the chemical that appears with it in greening potatoes, is. Solanine is most concentrated in the eyes, skin and sprouts of potatoes, which is why we often peel potatoes and remove their eyes and sprouts.

Q What do cats see when they look in the mirror?

A Cats see their reflection just as we do. Their eyes are similar to ours, so there's no reason to expect them to see anything other than their reflection. But how they interpret what they see is debatable. From their behaviour, it seems that they don't recognise the image as a reflection of themselves. Any animal that sees itself in a mirror or window will approach its reflection as if it were another animal. Cats tend to approach and touch the nose of the reflection. They are clearly confused by the resulting movement of the reflection, but they never seem to work out that it is their reflection.

Q Is it theoretically possible for the entire population of the Earth to stand on the Isle of Wight?

A No - it was once, but not now. The Isle of Wight is approximately 400 square kilometres in size. If you allow 625 square centimetres per person, you can fit about 1.6 billion people on the island. The world's population was about 1.6 billion at the turn of the century and didn't really exceed this amount until a few decades later. By 1950 it had reached 2.5 billion so it's possible this idea was put down in print at some point and has since became a popular myth. The world population now is about five billion.

Q Why do the bites of horse-flies hurt so much?

A Horse-flies don't suck blood like mosquitoes, whose mouth parts are highly modified for precision piercing and form a sharp, prominent proboscis which extends forward from the head. In horse-flies, the mouth parts are blade-like, and function as instruments for cutting through skin. The horse-fly literally bites you rather than injecting you, inflicting more damage to the skin, which is why it hurts more. While blood is oozing from the slashed skin, the horse-fly sponges it up.

Q Who invented the electronic pocket calculator?

A Texas Instruments of the US produced the circuitry for one in the late 1960s which was marketed by Canon of Japan, called "Pocketronic", first unveiled in Japan in autumn 1970. It could add, subtract, multiply and divide. But it weighed almost a kilogram, its display was on printed tape and it cost about $400.

Q Can acid rain and sulphur dioxide emissions actually be beneficial to plants?

A Yes - some plants react very well to increased levels of sulphur from the sulphur dioxide. Most plants' growth is determined by how much sulphur they get. But acid rain does more harm than good. It reacts with molecules in the soil causing the release of harmful chemicals such as aluminium ions and heavy metals. Sulphur dioxide can also diffuse directly into the leaves of plants causing instant cell damage.

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