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 neutralize a crowd without, uh, permanently neutralizing the crowd. Here's hoping you never get caught in cloud of tear gas, but if you do, now you'll know what's going on. Not that it'll be much comfort. [Wired]