What is surface tension ?

When a small quantity of water is poured on a clean glass plate, it spreads in all directions in the form of a thin film. But when a small quantity of mercury is poured on the glass plate, it takes the form of a spherical drop. Similarly, if a small quantity of water is poured on a greasy glass plate, it also takes the form of small globules like mercury. This shows that the behavior of liquids is controlled not only by gravitational force (weight) but some other force also acts upon it which depends upon the nature of the surfaces in contact. If the weight of the liquid is negligible then its shape is perfectly spherical. For example, rain drops and soap bubbles are perfectly spherical. We know that for a given volume, the surface area of sphere is least. Hence may say that the free surface of a liquid has a tendency to contract to a minimum possible area.