Explain the processes of headward erosion and stream piracy?

Answer 1

Headward erosion is a fluvial process of erosion that lengthens a stream, a valley or a gully at its head and also enlarges its drainage basin. The stream erodes away at the rock and soil at its headwaters in the opposite direction that it flows. Once a stream has begun to cut back, the erosion is sped up by the steep gradient the water is flowing down. As water erodes a path from its headwaters to its mouth at a standing body of water, it tries to cut an ever-shallower path. This leads to increased erosion at the steepest parts, which is headward erosion. (wikipedia)

When this happens, the erosion can break through into another stream and the water from the stream is diverted. So the stream is "stealing" the other stream's water.

Answer 2

Headward erosion is the retreat of a river's source through erosion at the head of the river. This process lengthens the river's course and can create valleys. Stream piracy occurs when one river captures the flow of another river through erosion, often as a result of headward erosion, diverting the stream's flow in a new direction.