What makes the earth rotate?
The earth rotates simply because it has not yet stopped moving. The Solar System, and indeed the Galaxy, were formed by the condensation of a rotating mass of gas.
Conservation of angular momentum meant that any bodies formed from the gas would themselves be rotating. As frictional and other forces in space are very small, rotating bodies, including the Earth, slow only very gradually. For the Earth rotating on its axis, there is no force working to counteract the rotation (except the tidal effect of the Moon, but that's working very slowly).


What is the peculiar smell of the earth after the first shower?
A pleasant smell after the first shower is because of a group of filamentous bacteria Actinomycetes found in the soil. They grow well in soil when the conditions are damp and warm.
When the soil is too hot, the bacteria are not able to tolerate the desiccation, so it produces spores as survival strategies. The spores remain invulnerable and are resistant to desiccation and heat. During the rainfall, the spores are taken up in the air by the force of wind and suspended in the air as aerosol. When we breathe the air, which contains spores, we are able to feel the earthy "after the rain smell". Geosmine (dimethyl-9-decalols) is the microbial product found in the spores is responsible for the pleasant smell.


Soaps come in different colors. But why is soap's lather always white in color?
Lather or foam is nothing but a large collection of small soap bubbles. A soap bubble is , in turn, a very thin film of soap solution enclosing some air. Because of the low surface tension of soap solution, the film can stretch and spread and form innumerable bubbles with a very large total surface area.
Due to this, whatever slight tint is present in the thin film of the colored soap solution gets subdued. Although a soap film is more or less transparent, the lather or foam looks white because the light striking this large collection of bubbles gets scattered. That is the reason why all kinds of lather or foam appear white.


How are some insects able to walk on the surface of water?
Water has a see-through film on its top layer that is created by surface tension. That means molecules of water are more likely to cling to other molecules of water than to something else. Some insects have a waxy coating on their body/feet. The surface tension of this coating [20 to 30 ergs/cm{+2}] is much less than water [72 ergs/cm{+2}] and hence water tends to bond to itself rather than wetting the insect's feet (in other words this wax is water proof). Secondly a sufficiently large amount of surface area of insect body is in contact with the water. The heavier the object, the more surface area is necessary to maintain floatation. As insects are very light weight the area of contact with the water surface is enough for it to prevent it from drowning.

Why cannot a petrol fire be extinguished by water?
Water, which is heavier than petrol, slips down permitting the petrol to rise to the surface and continue to burn. Besides, the existing temperature is so high that the water poured on the fire evaporates even before it can extinguish the fire. The latter is true if a small quantity of water is poured.

Why is it easier to swim in the sea than in a river?
The density of sea water is higher; hence the up thrust is more than that of river water.

Why does milk curdle?
Lactose (milk sugar) content of milk undergoes fermentation and changes into lactic acid which on reacting with milk protein (casein) form curd.

Why does water remain cold in an earthen pot?
There are pores in an earthen pot which allow water to percolate to the outer surface. Here evaporation of water takes place thereby producing a cooling effect.

Why are ventilators in a room always made near the roof?
The hot air being lighter in weight tends to rise above and escape from the ventilators at the top. This allows the cool air to come in the room to take its place.

Why does grass gather more dew in nights than metallic objects such as stones?
Grass being a good radiator enables water vapour in the air to condense on it. Moreover, grass gives out water constantly (transpiration) which appears in the form of dew because the air near grass is saturated with water vapour and slows evaporation. Dew is formed on objects which are good radiations and bad conductors.

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"Life is either a daring adventure or nothing."
Helen Keller