Put -er on mix and you get the one that does the mixing — and English never says who or what that is. So the word can land on a machine with blunt blades turning in a bowl, on a slow drum of cement on a building site, on a tap that brings hot and cold water into one flow, on a desk of sliders holding four sound sources in balance, on the bottle of tonic that goes into another drink, on the person who moves round a room joining strangers up, and even on the evening that was arranged so that joining up would happen. The picture to hold is simply: things were separate, and this is what brings them together. Everything around the word — the noun in front of it, the room you are standing in — tells you which separate things are meant.