> If someone was to ask me how far is was from sun to earth, and I answered 1AU, and they asked what an AU was? You'd probably do the same thing that you would

> If someone was to ask me how far is was from sun to earth, and I answered 1AU, and they asked what an AU was? You'd probably do the same thing that you would do if they asked what a km was: keep changing the units until you get one that they recognise, and then now they can understand the original unit you used. (Although in this specific case given that they're asking about the definition of the unit you'd probably provide that context without needing further prompting in the first place, similar to if they asked about the circumference of the earth and you gave an answer in metres)