Ball-tampering has once again become a big issue in cricket following the Australian side doctoring the ball in their test against South Africa. But why do players do it?

What is the law around ball-tampering?
The only action which may be applied to a cricket ball is polishing without the use of an artificial substance, drying with a towel or removing mud under supervision by the umpires. Anything else is illegal.

How do cricketers tamper with the ball?
There is a long tradition of fielding sides attempting to subvert this law by manipulating the ball with outside agents - and a continuum of offences.

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At the lowest end, players can use spit or sweat to polish one side of a ball and will often often suck sugary sweets or apply lip balm to aid this process. There is also a habit of throwing the ball into the wicket-keeper so one side lands on a rough, hard surface, scuffing it up in the process.

Why do players do it?

The motivation is to aid swing through the air. Conventional swing sees the ball move towards the heavier side as it travels through the air, so fielders will try and make one side 'heavier' by applying moisture to it (usually spit or sweat). Reverse swing sees the process switch around, with the ball moving towards the dry side.

Why this happens - and when - is a source of debate within the game but players will try and aid this process by keeping the dry side very dry - scuffing it can also produce more swing.

Are Australia the worst offenders?
No. There are manifold examples of players being punished for ball-tampering, from a range of countries. England captain Michael Atherton was fined £2,000 for applying dirt to the ball in a Test against South Africa in 1994 - he denied ball-tampering, saying that he carried dirt in his pocket to dry his fingers - while Pakistan refused to re-enter the field following a tea break after being accused of tampering in a Test against England in 2006, a charge which was later dismissed by the ICC.

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South African captain Faf du Plessis has twice been found guilty of tampering, while fast bowler Vernon Philander was also deemed to have changed the condition of the ball by using his finger and thumb to scratch it.

What are the punishments?
The official punishment is light: at the time, umpires can impose a token five-run penalty against the bowling side and the replacement of the ball.

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The offence is deemed to be Level Two, two below the most serious level. Possible sanctions include a deduction of a player's match fee anywhere between 50 and 100 per cent, and three or four demerit points added to his record. Four demerit points triggers a one-Test ban.