The first summer job I ever had was mowing lawns. Back then (this was the ’70s), I would start the power mower, and the blade kept spinning until I turned the mower off. I learned quickly to pull the mower up a slope and not push it, because if you slipped, the mower would roll down onto you, possibly causing a serious injury. One of my friends mangled his toes when a mower ran over his foot. Nowadays, every lawn mower has a safety bar that you have to keep squeezed against the handle, otherwise the engine will shut off. If you happen to let go of the mower, it will stop. The safety bar has no doubt cut down on the kinds of injuries I had to worry about. In human factors terminology, that bar is a constraint; it helps prevent the user from doing or experiencing something undesirable. At the same time, it maintains the affordance of a lawn mower; it allows the user to cut the grass.