 Oh hello, you're back. So in one of the earlier videos we talked about the fact that chemistry is interested in measuring or learning about features or properties of material objects. Because chemistry is interested in learning about those features, chemistry has to make measurements about those features. So believe it or not, we are going to have to spend a fair amount of time talking about measurement and how to measure things and the good and bad parts of that and that probably sounds like an incredibly boring topic. And that's because it is, but you're stuck with it. And so let's just keep going. So chemistry measures features matter, we need to talk about measuring things. One thing to point out is that measurements are often numeric. What I mean by that is when you make a measurement of something, you usually make the measurement and get a number out. So if you measure how tall something is, you'll measure it almost always numerically. So as an example of that, if I went up to you and I said, I am 71.5 tall, I'm giving you a number here, which is a numeric part of the measurement. There's a problem with the statement that I wrote. It's probably pretty obvious to you. You don't know what I'm talking about with respect to 71.5. 71.5 what tall? Am I 71.5 bologna sandwiches tall? It's not clear because I'm not giving you any additional information except for that number. You need additional information to make sense of the 71.5. So most of you probably know what's coming next, but just in case you aren't, what makes this statement a little more understandable to most people is if I give you the number, I say I'm 71.5, but then I give you another piece of information right next to that number. I say I'm 71.5 inches. So whatever an inch is, as far as length is concerned, I am 71.5 of them tall. The inch has a fancy name. It's called a unit. It is an example of a unit. And we are going to talk about units a fair amount. So what is a unit? Whenever we make a measurement, I can't really think of any exception to this. Your measurement will need some sort of unit. So what is a unit? A unit is a specific amount that people come to an agreement on, and I'll try to explain that in a minute, a specific amount of some certain feature that we can actually measure and that we can all, everybody see the all here? We can all use it as a common reference. So the way that I want you to think about inches or any other unit is the inch was basically created by a committee of people who got together and said, look, everybody is on a different page when we're trying to describe how tall things are to each other, and we don't want to be confused anymore. So let's come up with a common distance or a common measurement of length, and I don't know, maybe it'll be about this long, and we're going to call that one inch. And as long as everybody agrees to make their measurements in inches, then nobody's going to be confused anymore, or there will be much less confusion than there is at the moment. So an inch is an example of a unit, and it's a specific amount. It's this much of some feature, in this case the feature that we're talking about is tallness or length, that we can actually measure, hopefully you can measure this, and that we can all use it. We all come to an agreement because there was a committee that came to an agreement and said, this is what an inch is going to be, and a lot of people should use this for measuring the length of things. So it's not just an inch that can be a unit, it's any type of thing that you may want to measure. So an example of another thing that you may want to measure is how much something weighs, and one example of a unit for that is called a pound, which you're probably familiar with. I'm sure some committee out there has gotten together and made a definition for what a pound is so that everybody's on the same page, but basically when you have a unit, then you can make measurements that everyone can compare against and everyone can hopefully understand. So that is what we are going to talk about a fair amount in the upcoming sections.