If you have a mouse that has a ball underneath rather than a red light then you are going to have to clean it every now and then.
A lot of computer users get confused when the mouse pointer starts to stick, move in jerks, or fails to move at all despite them running the mouse around the desk like a maniac.
What has happened is that the rollers that the ball presses on to convert the movement into electrical signals have got covered in grease and dust and the ball is slipping on them instead of rotating them.
Turn the mouse over and you will see a plate covering the ball which can be unscrewed by applying pressure with a finger to each side and turning anti-clockwise about a quarter turn.
Turn the mouse back over and the plate and the rubber ball should drop into your hand.
Inside the ball cavity are two long rollers that will be covered in a certain amount of gunge and grot! Look at the spot in between the two rollers and opposite that spot will be a small spring-loaded roller that keeps the ball pressed to the two larger rollers.
Now it's a matter of scraping off the covering of grease and dust, rotating the rollers a little at a time until they are clean all the way round. You could be slightly amazed at the amount of greasy fluff you have to remove! It is caused by the mouse rolling over areas where you have previously rested your hand and the skin's natural moisture mixes with dust to form this material. Clean the small spring-loaded roller too - this is a bit more difficult because the pressure of your finger makes it retreat into the body of the mouse against its spring!
Once you have cleaned the rollers, use a small paint brush to clean the ball cavity.
The ball itself probably won't need cleaning but in extreme cases it can be cleaned by brushing it with a slightly stiffer brush. Don't be tempted to wash or wet it. Clean Blue Tack can be useful to pick off bits, but make sure you don't leave any shreds of Blue Tack on the ball.
Before putting the ball back, clean off any debris and grease from the underside of the mouse casing - the feet and around the edges of the label.
Lastly drop the ball back in, taking care not to handle it unnecessarily - you'll just transfer more grease onto it - and then place the retaining plate back over it, aligning the lugs and twisting clockwise to secure it. Your mouse should now work perfectly for another couple of weeks or so!