You know how scissors work? Two opposing blades close on a surface and create shear stress on a localized area, cutting the surface in the process. A lawnmower blade is different: Just a massive and rather dull blade with a lot of inertia going around a fixed axis. How can such a blade cut grass since grass is super lightweight and kind of just moves out of the way when the blade passes over it? Find out in this episode of “Smarter Every Day.”