Ask Uncle Pai — Why does our body itch?

Where do stars go during the day?--Renita D'sa, PrabhadeviThe stars do not go anywhere. They are very much there in the sky. If someone wer...

Where do stars go during the day?
–Renita D’sa, Prabhadevi

The stars do not go anywhere. They are very much there in the sky. If someone were to look up from the bottom of a well, he would see stars even during the day.

Why do we see lightening much before we hear the thunder?
–Parinita Parameshwar Devadiga, Bhandup (East)

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Answer to this question is to be found in school text-books. Light travels at a speed of about 3,00,000 kilometres per second and sound,

at a speed of about 332 metres per scond (in the air). This speed is much slower than the speed of light. Naturally you hear the sound much after you see the lightening.

Why does rainbow have only seven colours?
–Suma, Udipi, Karnataka

The rainbow is caused by a phenomenon called refraction of light. Each drop of water acts like a prism. When light passes through it, and again as it enters the air from within the drop, the light breaks down into many colours. Of these, the human eye can see only seven colours — violet, indigo, blue, green, yellow, orange and red. The colours below red are referred to as infrared and beyond violet are referred to as ultraviolet. These cannot be seen by the human eye.

Why do the blades of an electric fan keep on rotating for some time, even after the current is switched off?
–Bento Fernandes, Andheri West

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When you learn physics in higher classes, you will come across the Laws of Motion, by Isaac Newton. The first Law of motion states that every body, continues in a state of rest or uniform motion, unless compelled to do otherwise by an impressed (external) force. According to the Law, the blades of the fan should keep on rotating, once they start rotating. They stop because of the resistance offered by the air to the movement of the blades.

All animals, birds and even human beings scratch parts of their body to get over with the irritation. But what causes the irritation? Is it caused by a germ or bacterium trying to enter our body?
–S R Bhattacharjee, Nagpur

Irritation is not caused by a bacterium or a germ trying to enter the body. You might have read that waste products are thrown out of the body through faeces, urine and perspiration. In summer we perspire a lot. Most of us suffer from prickly heat in summer, particularly in humid places like Mumbai where the atmosphere is loaded with moisture, which slows down the process of evaporation of perspiration. The pores in the skin get clogged, as a result, and waste products accumulate on the skin to cause itching. When the blood circulation is good, wastes are carried away in the veins and fresh blood is brought through the arteries and its capillaries. Blood circulation near the area of a scratch is generally affected and can lead to an itch.