There's been some confusion over the size of today's explosion in China's Tianjin province, with some people mistakenly thinking it was the largest non-nuclear explosion of all time. It was not — that honour goes to the Russians — and you can partly blame our stupid system of measurement for the mistake.
An explosion is a release of energy, and would probably best be measured in a unit of energy, like joules. Instead, we measure the size of the explosion based on how many tons of TNT it would take to produce a similarly-sized explosion: when the news talks about a '7-ton' explosion, it means that seven tons of TNT would produce the same magnitude explosion.
This is a bad unit of measurement for a bunch of reasons. It's arbitrary — why pick TNT over dynamite? — and also misleading: one gram of TNT, or trinitrotoluene, has an explosive energy of somewhere between 2000-6000 joules, depending on the composition and density. To make it more useful as a quasi-scientific unit, that energy level was fixed at 4686 joules per gram, making an arbitrary standard even more so.
Explosions also don't generate a consistent type of wave: an explosion of a block of TNT, for example, behaves very differently to a fuel-air bomb, which in turn behaves differently to a meteorite impact; yet, all are measured relative to a chemical compound discovered by a German chemist two centuries ago.
Worst of all, the way people say it — as a 'five-kiloton explosion' — is confusing and awful, because we already have a unit called the ton. Only, that one isn't connected to explosives. Let's come up with a less random and more scientific unit of measurement for explosives instead. And, if we could kill the imperial system while we're at it, that would be great.