 Most measurements when you're trying to be precise look at the world very carefully come down to a number So for example, we might ask how tall are you Joe? This is a picture of Joe I'll write that there so that you can tell and to answer that question you do a measurement with a measuring tape or something similar Maybe a laser finder whatever tool you have and you might get the answer six now if you're looking at answer six There's an obvious really really important piece of information missing the units What are the units of this number and any time you make a measurement? It's going to have some kind of units and those units are really important So in this case I was six times the height of a Scottish Terrier and that unit is called a rough So six roughs, which is approximately equal to eight meows, which is the height of a Cheshire cat Now I must admit that roughs and meows are fairly unusual units of length In fact, I kind of just made them up, but at least their units of length I couldn't possibly have been six minutes tall. I couldn't possibly have been eight kilograms tall