Why are the two sides of a coin known as heads and tails?

Generally, one side has the imprint or embossing of the official head of state or an insignia or emblem and hence it is called the head side. It is the main side of a coin carrying the portrait head of the ruler or a Symbol and name of the country. Since the opposite of the head is the tail, the reverse side is naturally called 'tails'. It depicts the denomination or issue price of the coin with year and mint mark underneath. 

Technically, the heads and tails sides are known as the obverse and reverse, respectively. The tail side name possibly originated from the British ten pence coin, the reverse of which shows a heraldic lion with its tail raised.

Why does sugar dissolve faster in hot water than cold water?

Sugar dissolves by water molecules surrounding sugar molecules and separting in from the rest of the solid. When sugar is added to water, energy is used up to break apart the molecules in the sugar. The water molecules then begin to spread out to make room for the sugar. This way the sugar gets dissolved in the water.

When the water is hot, the sugar molecules break apart faster, and the water molecules move around the sugar faster. With increased temperature, the water molecules are moving more rapidly, increasing the speed at which hydrated sugar molecules are transported away from the solid, increasing the rate at which the solid dissolves. In cold water, this process is slow, hence the sugar does not get dissolved faster.

Why do we sweat on a hot day?

When the body temperature rises, the sweat glands are stimulated to secrete perspiration. It is nature's way to keep the body cool. During the process of evaporation of sweat, body heat is taken away, thus giving a sense of coolness.

Sweating is therefore the human body's primary cooling mechanism. Because this mechanism uses water, we need to replace lost fluids by drinking more fluids in hot weather. This is especially true after exercising or working in hot weather.  When we sweat, our skin and clothing become covered with water. If the atmospheric humidity is low, this water evaporates easily. The heat energy needed to evaporate the water comes from our bodies. So this evaporation cools our bodies.

Why do owls have three eyelids?

Owls are equipped with 3 eyelids. They have a normal upper and lower eyelid, the upper closing when the owl blinks, and the lower closing up when the Owl is asleep. The third eyelid is called a nictitating membrane, and is a thin layer of tissue that closes diagonally across the eye, from the inside to the outside. This cleans and protects the surface of the eye.

Why does a wet cloth appear darker than when dry?

We see a cloth's colour by the light which hits it. For example, when white light (containing all colours) hits a green shirt, all the colours in the light are absorbed by the shirt,except green, which gets emitted. This emitted green light is why the shirt looks green to our eyes.
When the shirt becomes wet, water is absorbed in the fabric, including the outer layer of the material. When white light hits the surface of the material where it is wet, some of the light will scatter (refract) from the water. This light doesn't make it back to our eye. Less light coming from the wet region means it will look darker. Not all of the light that is emitted by the green fabricreaches your eyes now. Because there is less light coming from the wet area (some was refracted in a different direction), that area will look darker than the material around it.

Why water droplets are formed outside a glass containing cold water?

The cold water reduces the temperature of the glass. The cold glass reduces the temperature of the air around the glass. The moisture in this surrounding air gets condensed into liquid droplets. Hence we see a layer of water droplets on the outer surface of the glass.