Explain why objects moving in fluids must have special shapes ?

Explanation:

Fluids exert both drag and lift forces on moving objects.  Drag is the frictional force opposing motion.  Lift is the force perpendicular to motion.

Some objects, like parachutes, are designed with large cross sectional areas to increase drag force.  Usually though, objects are designed to minimize drag force.  It's why cars, planes, and boats have sleek shapes.

Airplane wings have shapes called airfoils that generate lift.  It's what makes them fly.  The same shape is found in racecar spoilers.  These spoilers use lift force to push down on the rear tires, increasing traction.