It's a trick a buddy showed me when I was a kid, his father showed him etc etc. See before we had a billion parts stores, you would often have to make a gasket. So the founding fathers figure out...that if you lay a roll of gasket paper on something hard with a sharpish edge and use the round side of a ball peen hammer to tap the gasket where the edge is located, it'll actually cut you the perfect gasket. Very cool thing that works well.