 So if we do have an area that we've defined, sometimes this area is called a control volume, as long as it's assumed that the volume is the same. But if we do have some sort of volume inside of our boundaries, and then we have a change in that volume represented by what's flowing out of it, or potentially into it, there's some sort of change to this volume. And this change must happen in a finite amount of time. So it must occur in some period of time. Well, if we take that amount of volume, and we divide it by the amount of time, that is the definition of what we call flow. And the letter we typically use for flow is the letter Q. And usually the units for something like flow are going to be in units of volume over time. For example, it might be something like meters cubed per second. That would be an example of volume over time. Or we might have feet cubed per second, or feet cubed per year, if we're talking about very large time frames. Obviously that's probably a very large number if it's talking about something accumulating over the course of a year. So those are examples of units of flow. So let's consider an example. Here I have a kiddie pool. Okay, over here I have a kiddie pool. And I'm going to take my garden hose and use it to fill my kiddie pool. And the dimensions of the kiddie pool, we'll say it has a diameter of six feet. And the question we want to know is how long is it going to take to fill the kiddie pool? Let's say the kiddie pool only needs to be filled to a depth of the water of one foot. Well, in order to know that, we'll need to know, we'll need to look up, perhaps, some information about how fast my hose is flowing. And we're going to say that the hose supplies an amount of 0.8 CFM. CFM, we'll CFM what might that stand for. In this case, that stands for cubic feet per minute. In other words, feet cubed per minute. So let's look at our relationship here. We have our relationship sort of written backwards here of Q equals delta V over delta T. Let's reorganize that relationship a little bit. I want to know how much time it takes to do something. So if I reorganize that, let's see here, we'll end up finding out that the amount of time it takes to do something is equal to the amount of the change in our volume divided by the flow. Notice these are same relationships algebraically. Okay, well, I can go ahead and plug in my values here. The change in the volume, since we're filling the pool, we're assuming that the volume starts at 0. So it's actually the total volume of the pool. Well, in order for me to figure out the total volume of the pool, let's see here, volume of the pool is equal to the area of the pool times the height that we're going to fill the water to, or the depth of the water. Well, that area is a circle, so we're going to have to go ahead and do pi r squared, pi r squared times that height. Oops, the area of that circle is equal to pi r squared times that height. Well, if I look at my diameter of the pool of 6 feet, there's the diameter of the pool of 6 feet, if I look at that diameter, divide that in half, that's 3 feet. And if I plug that in here for the radius, we get a value of 9 pi times the height of 1 feet. So that's 3 times 3 times 1, 9 pi cubic feet. And then if I multiply by pi, I believe the number I get is 28.3. So there's the volume of the pool, the amount of water I want to put in there. And I want to go from 0 to that 28, so that would be the change in my volume. If I plug that in here, the amount of time it takes me to fill the pool is going to be equal to that 28.3 cubic feet divided by my flow rate of 0.8 cubic feet per minute. Notice the cubic feet cancels out. The minute since the denominator in the denominator goes up to the top and we get our answer in unit of minutes and we get an answer of 35.8 minutes. So with the somewhat typical flow of about 0.8 cubic feet per minute, with that somewhat typical flow, we'd find it takes us about a little under 40 minutes, 35 minutes or so, to fill the pool.