Question

a drop of liquid assumes a spherical shape ?why?

Answer 1

A drop of water assumes a spherical shape due to surface tension.

Answer 2

    A small liquid drop assumes the shape of a sphere, because of the surface tension force on the surface.  The liquid tries to pull in all the liquid on the surface towards inside.  So the liquid tries to occupy the minimum surface area for the given volume of liquid.

    Sphere is the shape that has the minimum surface area for a given volume. The spherical shape is a little distorted, prolonged or bulged at the bottom due to weight of the drop.