Ball tampering

Since cricket began bowlers have tried everything to encourage the ball to do something different - to swing in the air or seam off the pitch for instance - and hence give themselves a better chance of taking a wicket.

The Laws do permit fielders to polish the ball providing that no artificial substance is used and that the polishing doesn't waste any time. They are also allowed to dry a wet ball on a towel, and they can clean mud from it - under the supervision of an umpire.
Ball tampering is the commonly-used expression for what the Laws call changing the condition of the ball and refers to the things that fielders might do to the ball that are most certainly prohibited. What they are not allowed to do is anything else that could alter the condition of the ball, such as rubbing it on the ground, interfering with its seams or scratching its surface. Such actions are regarded as unfair play - cheating. Umpires are instructed to make frequent inspections of the ball at irregular intervals throughout the match. If they agree that the condition of the ball has been unfairly changed they should, firstly, change the ball for another that has equal wear, award 5 Penalty runs to the batting side, inform the fielding captain of the reason and, when circumstances permit, tell the batting captain and make a report to the appropriate authority.
If it happens again the umpires should repeat the entire procedure with the additional punishment of the bowler concerned being taken off and not allowed to bowl again in that innings.