Whatever you do, you are using your muscles. Not all of them move at once you have 639 muscles in all but you would be surprised to know how many of your muscles you use doing something as simple as walking. When your muscles are working they produce an acid called lactic acid, which makes the muscles feel tired. When you rest, your body gets rid of the lactic acid, and you don’t feel so tired. When you do a lot of hard exercise, like running a race, you may get cramp, a sharp pain in the muscles you are using the most. Cramp is caused by a big build up of lactic acid in these muscles, and lot of rest is needed to get rid of the cramp. So although exercise, as we know, is good for your body, rest is just as good, and just as important.