True story: I almost got hit by a car while doing a fartlek run one time. Why? Because I was laughing uncontrollably, and crossed the road carelessly. The driver stopped, but I kept laughing. Why was I laughing like a loon mid-run? Because I was doing a fartlek run, and farted on the fast speedup, and couldn't get over the irony. :) This story has nothing to do with the meaning of fartleks, but it's kinda funny, and I know you can relate (as a fellow runner). But despite it's odd name, the fartlek run is an incredibly powerful tool