Alcohol.
Call it ethanol, ethyl alcohol, grain alcohol—we don’t care. We do, however, care very much about how one measures its concentration. An evening spent in a pub is often soured when fellow drinkers start bragging about the “140 per cent proof” rum they brought back from Cuba. 140 per cent? To deconstruct that sentence, every 100 mL of the rum being drunk contained 140 mL of ethanol, and then the word “proof” is appended as some kind of catch-all mathematical-abberation nullifier. Preposterous.
The correct sentence would contain the description “140 degrees proof”. To get from there to a percentage value, one can either side with Americans, and divide by two, to give a 70% concentration, or keep things old-school and revert to the 4/7 ratio of Olde England, to give an 80% alcohol concentration. Or use this handy calculator. Which do we prefer? The 4/7 degrees/per cent, for two reasons. Firstly, any measure defined by an ability to ignite gunpowder has to be the correct one, and secondly, taking the time to multiply a number by 4/7 in the middle of a heated debate—often whilst drunk—is the true mark of the pedant.
