It's called a stitch, and while it's not as painful as, say, a root canal or, em, labor, it is a nuisance. And, according to Men's Health, it's entirely preventable. It's caused by putting pressure on the right side of your body -- where your liver is -- because most of us tend to exhale when our right foot hits the ground. But if we make one small change -- exhaling when our left foot hits the ground -- stitches may be far less common.

It's an easy switch, but chances are you'll have to do some careful thinking when you head out for your morning run.