Due to the cohesive forces between the molecules of a liquid, the free surface of a liquid drop always tries to contract to the smallest possible area. As a result the liquid assumes a spherical shape because for a given volume, a sphere has the minimum surface area.

When a liquid drop is placed on a horizontal surface, there are two forces acting upon the liquid drop, one is the force due to gravity and the other is the force due to the surface tension of the liquid. The force due to gravity tends to flatten the liquid drop until its surface becomes horizontal and the force due to the surface tension tends to decrease its surface area and tries to gather up into the form of a sphere because sphere has the least surface area for a given volume. For a small drop of the liquid, only the force due to the surface tension is pronounced. So for this reason, the small drop of liquid is spherical in shape.