A lot of us remember these from when we were growing up. Officially referred to as exercise-related transient abdominal pain (ETAP), the causes of side stitches and side aches aren't fully understood by doctors. That's too bad, because up to 60 percent of runners deal with the pain in any given year, according to some studies [source: Morton].
Like many ailments associated with youth, this is one that, for most people, decreases in frequency and intensity with age. Stress on muscles in the abdomen is generally accepted as the cause, though which muscles and what stresses them is a matter of some debate.
To beat this annoying pain, avoid eating or drinking too much before a run. That doesn't work with everyone, so you may have to experiment to see what works best for you. Resting for a bit usually relieves the pain. When you start up again, you may have to go slower to keep the pain at bay for the rest of your run.
Just be glad that getting older is a positive influence on saying goodbye to side aches for good.