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Police in the US use firework-like stun grenades, known as “flash-bangs”, to disorient hostage-takers before rushing in to capture them. The canisters—which are loaded with metal powder and solid oxidiser—are not high explosives, but they can injure someone if they go off next to the head. Now Sandia National Laboratories in New Mexico has developed a safer alternative. Pulling a grenade-like pin on the plastic canister ignites a couple of grams of a black powder propellant, producing hot gas that sprays fine aluminium dust through 16 holes in the canister to form a cloud 1.5 metres-across. The hot gas and a few hot particles ignite the dispersed aluminium dust, which produces a…