 The universe is a large and complicated place, and humans have been trying to understand it ever since they've been around for millions of years. And for the last few millennia, we've been working particularly hard, building tools like mathematics and writing and education systems and philosophy and trying very much to understand the details of the world around us. But things changed a few centuries ago with the advent of science, and the difference between science and the other attempts to understand the universe is that in science we try and follow evidence. We don't follow authority what people think, we go out and look if we have an idea, we examine the evidence, and examining the evidence means we have to make a measurement. So central to all science is the idea of measurement. Most measurements when you're trying to be precise and 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 6. Now if you're looking at answer 6 there's an obvious really really important piece of information missing, the units. What are the units of this number?