When we cut a watermelon into slices, we are converting a solid shape into other solid shapes. Regardless of the size and shape of the slices, there is one fact that holds true of the whole process. The volume of all the slices together exactly equals the volume of the original watermelon.
When we convert a solid of a given shape to a solid of another shape, the surface area usually changes. However, the volume is preserved.

Volume of object1 = n x Volume of object2

surface area of cuboid = 2 {(l x b) + (l x h) + (h x b)}

Volume of the metal sphere = n × Volume of a marble     

Volume of copper cube = n × Volume of the pipe 

Volume of cylindrical vessel of radius r2 and height h = Volume of the spherical ball

Volume of the embankment = π(r₂² – r₁²)h