Metals can be classified by hardness, malleability and conductivity. One quality you won’t find listed in the reference books is sparkiness.
A delicate balance between flammability and hardness determines which metals spark. For example, magnesium is a famously flammable metal, but grinding it produces no sparks because the energy needed to cut chips from the soft metal is not enough to heat them to their ignition point.
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.