 Let's do another example. If you know a cricket field, how much will the cut grass weigh? Okay, so once again, we have to guess several things in this problem. So we have a cricket field, large amounts of grass in it. And the tricky thing with any kind of estimation problem is trying to figure out something you can guess reasonably well. So going to make some kind of sensible guesses about how big a blade of grass is. So a blade of grass is maybe that long once you cut off. It's about that wide. And how thick will it be? Let's say it's about a millimeter thick. So our lengths will be something like three centimeters, maybe four millimeters, and maybe thick, we'll just make it one millimeter. Okay, so we have the rough dimensions of a piece of grass. And so if I look at the volume of that grass, I'm going to have, I've got centimeters, millimeters and millimeters there. And that's going to be rather confused. So I'll turn all of that into meters. And that'll let me cancel my millimeters by millimeters, my centimeters of these centimeters. And I've got my answer, 12 by 10 to the minus eight meters cubed, or more conventionally, 1.2 by 10 to the minus seven cubic meters. That's per leaf of grass. Now, if I want to know how much that weighs, I'm going to know its mass and its mass I can get from the density in the volume, which is just the definition of density really. And I'm going to assume that the density of grass is about the same as the density of water. So that's about 1000 kilograms per cubic meter. So that says that a blade of grass weighs something like 0.1 of a gram, which sounds vaguely in the right ballpark. It's very hard to be accurate about these things. But if I get that within a factor of 10, I should be okay. So now I have to figure out how many blades of grass do I have in my cricket field. Now I don't know how many blades of grass that might be in a huge area, but I can probably guess with some kind of accuracy for a small area. So let's imagine we have an area maybe just five centimeters on the side. So in a patch that big, I would expect there to be something like 50 blades of grass. But of course, a cricket field is much bigger than that. This is something more like 200 meters. And so if we think of it as maybe 200 meters by 120 meters, then the area of a cricket field is 24,000 square meters. The area of our little patch here, which remember we said had about 50 blades of grass, that area was five centimeters by five centimeters, which remember we're going to have to convert into square meters. So I'm going to get a factor of 100 from the centimeter and then another factor of 100. So it's going to be by 10 to the minus four square meters, which in the more normal way of running is 2.5 by 10 to the minus three. So if I have 50 blades of grass in this amount of area, then in that amount of area, I'm going to have a really very large number of blades of grass. And so if I want to find the total mass of those, I simply multiply the number of blades by the mass per blade. So you get 60,000 kilograms, 60 tons of grass. That sounds high to me. So it's not clear that we've done that well. We might find that we've overestimated the blade of grass's size significantly, or that we've overestimated how many blades of grass are in that patch significantly. Or it might be telling us that we really do need a truck in order to carry all that grass away. And so what we've discovered is that this is a potentially serious amount of grass. And then we should be more careful about our numbers. And so it didn't give us a tiny number or a huge number. And so all right, it matters whether we bring a wheelbarrow or a truck. Okay, let's go back and get our numbers accurate.