When the ball becomes very old around 40 or more over old, it can begin to swing towards the polished side; this is known as reverse swing. Reverse swing is difficult to achieve consistently, as it relies on uneven wear of the ball, tends to occur mostly in hot, dry weather conditions, and requires bowling at high speed. Normal swing can be achieved at moderate bowling speeds, but only the fasted bowlers can regularly produce reverse swing. Notable practitioners of reverse swing have mostly been Pakistani fast bowlers. Sarfraz Nawaz is generally credited as the first bowler to have used reverse swing. He passed on his knowledge to the legendary bowler Imran Khan.