Ever say, started sprinting for a fair distance without warning? (e.g. to catch a bus that was a long way off, or to get to a place you were late for) and got a sharp pain in your side while running that forced you to slow down or essentially stop? It usually occurs if a.) you've began running without warning, and so your breathing isn't paced properly or b.) you've had a big meal or drink before hand. That is the wonder of the stitch