Explain why objects moving in fluids must have special shapes.

The frictional force exerted by fluids is also called drag or fluid friction. To overcome or minimize air fluid friction, the bodies of objects minimise its surface i.e. sometime spherical shapes or other streamlined shape with smooth surface. Aeroplanes, jets, rain drops have streamlined curve surface to reduce air drag.

https://www.tiwariacademy.com/ncert-solutions/class-8/science/chapter-12/