Shear nuts are a security nut that are very difficult to remove without specialist tools, as their name implies the head of the nut shears off leaving behind a dome shaped nut which is very difficult to grip, making removal very difficult. The shear nuts used in this video are stainless steel M10 and require considerable force to shear the hexagon part from the body of the nut, these cost about £1 each. You can also get cheaper glavanised versions, they come in several bolt diameter sizes from M6 to M16.
Make a table clock from metal washer without welding.