What causes a "side ache" when running?

Question

I remember getting these a lot as a child and have noticed a few when running lately.

Answer 1

Its a spasm of your diaphragm (the muscle mainly responsible for breathing). If you get this in the future then push into your abdomen right underneath your ribs on the side you have the pain or try breathing against your body (when you breath in suck you belly in and when you breath out extend your belly).

Answer 2

Do you mean a 'stitch?' This is caused by not warming up correctly before running.