Why smoke of cigarette has blue appearance?

This is because of Tyndall effect. According to this effect when the light passes through medium containing small particles, it scatters, if the diameter of those particles is less than 25 nano meter the scattered light appears blue. This is the case of tobacco smoke of the cigarette.

Why coffee cools when you blow on it?

When you blow on your cup of coffee evaporation takes place. In the evaporation the water molecules leaves the water surface and comes in the air. In this process the escaping water molecule has to overcome the force of attraction applied by the other water molecules of the surface. As we know that the force of attraction between the molecules is of electrostatic type. So escaping molecule must have enough kinetic energy to overcome the potential energy of this force of attraction. Since leaving molecules possess more energy than the ordinary ones. The left ones are of less energy than the average. Hence the random motion of molecules decreases. As you may know higher the random motion of molecules higher the temperature of the substance . So temperature of liquid decreases.

Why high tension electricity lines sounds like sunnnn...... sunnnnn?

This is because when the voltage of A.C. are at its maximum value the electric break down of air occurs. And at the just next moment voltage is at its zero, so break down  is checked. This continuous process results this peculiar sound.  

How a match box works?

Red phosphorus is used in the striking strips on match boxes. When a match is struck, potassium chlorate in the match head mixes with some red phosphorus on the striking strip, and the friction is enough to ignite this mixture.

How invisible ink works?

Cobalt(2) chloride hexahydrate, dissolves readily in water to give a red solution. If we use this solution as an “ink”, we can write secret message on paper. The writing is not visible when the water evaporates form the paper. When the paper is heated, however the message can be rad.
If you don't want to be entangled in the science and just want to write with such magic ink, just use juice of onion as a ink. To read the massage again heat the paper. And enjoy yourself !

For an another interesting trick, go to the page 'Fun Creation'.

Why plaster of Paris used in designing the houses.

When POP is added to water, it forms a thick slurry that can be poured into a mold. As it takes on more water, the material increases in volume and forms a hard, inflexible solid. This property makes palster of Paris a useful material for artists, because the expanding compound fills a mold completely and makes a high quality reproduction.