DUE TO SURFACE TENSION WATER MOLECULES TEND TO HAVE MINIMUM SURFACE AREA AND SPHERE HAS THE MINIMUM SURFACE AREA FOR A GIVEN VOLUME AND THEREFORE BUBBLES ARE IN SPHERICAL SHAPE ONLY.
The Bubble''''s shape is spherical so as to reduce the surface tension. With the given volume of liquid,Spherical surface only fits i minimizing tthe surface tension to a greater extent than other shapes viz. cylindrical,cubical etc. and hence the shape is spherical