 So we've already discussed the importance of making careful measurements in science, and recording not only what value you get, but how sure you are about that value. And important discoveries have often been made by looking very carefully at measurements and noticing small discrepancies from what we expected, and those small discrepancies eventually led on to big discoveries. But sometimes just making a good old guess at what something should be is an incredibly powerful tool. And the reason for that is that when we're trying to figure things out, the most important question we start with before we start doing our calculation is, well, what's important? What things should I consider for this calculation to figure this thing out? And figuring out what's important often involves thinking about possibilities and then estimating to see whether they're relevant or not. Sometimes they'll be really important, but often they won't be. And that's where estimation will be a very powerful tool in getting you started on any kind of real world problem.