Why to open the cap of a fountain pen is much more easier than the back, although both the threads are of same pitch and have same number of turns?

This is because when we open the fountain pen to refill the ink, the thread is a single start thread but the thread at the cap and pen joint is a multi-start thread.

Why do the skin of my finger tend to get wrinkles when my finger get too much water?

Some of the cells in the skin of your fingers, called "keratinocytes", self-destruct naturally while your skin is developing. This causes the development of layers of cell membrane and keratin-rich cellular components in your skin. When your hands get wet for a long time, moisture is able to get into these layers if any openings are exposed (very normal due to how skin wears out) and is absorbed by the keratin-rich molecules. This causes a temporary swelling of these layers. The skin above it then starts to wrinkle because it's on top of the swelling layers. If left to dry normally, the water will evaporate from the same places that it got into these layers. This is helped by your body-heat essentially burning off the liquid.

Source(s):

http://www.scientificamerican.com/article.cfm?id=why-do-fingers-wrinkle-in

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'.