Tear gas is a relatively modern invention -- a non-lethal repellant developed from the killer chemical weapons of World War I. But each canister relies on the fascinating chemistry of some ancient compounds and reactions. Wired gives us a look inside, without all the coughing and choking.
Turns out there's a tricky balance in the equation. If the reaction goes too fast, things will get explodey, and each canister has to pack enough gas to neutralise a crowd without, uh, permanently neutralising the crowd. Here's hoping you never get caught in cloud of tear gas, but now you'll know what's going on if you do. Not that it will be much comfort. [Wired]