Explanation:

Surface tension

We all know that for a given volume of a liquid, a sphere has the least surface area. Also, the more the surface area of an object, the more is its surface energy. 

Now, to remain in stable equilibrium, a body always tries to minimize its surface energy. Therefore it tries to reduce its surface area and hence acquires a spherical shape. 

All this phenomena is because of the presence of surface tension of a fluid.