Question

 An electric motor generates electricity if rotated manually. Why?

Answer 1

 An electric motor can generate electricity only if it is rotated at a speed that is greater than its synchronous speed. So it doesn't matter if you rotate it with water turbine. You just have to rotate it at a greater speed than its synchronous speed.This is because of friction that is used to burn matchstick

Answer 2

An electric motor and an electric generator have almost identical structure and parts. Motor is used to convert electricity into mechanical energy. Generator is used to convert mechanical energy in to electricity.

Generator shaft is rotated by (potential energy of) water or steam energy (work done by pressure), then changing magnetic flux through the armature coil generates AC current, as long as the shaft rotates.

In Motor the shaft wound with current carrying coil is rotated by the magnetic force on the current carrying armature coil, due to change in magnetic flux.

Same principle and same parts in both. Motor and generator can function as both.