 And in fact, there are lots of relationships between all these different units. For example, a Newton is a unit of force, and force, we will find, is equal to a mass times an acceleration. Which is the same as a mass times a distance divided by time squared. And so the unit has got to be kilograms meters per second squared. And indeed, one Newton is exactly one kilogram meter per second squared. So if I know what something is, if I know that a force is a mass times an acceleration, then I can figure out that the unit for force must be a kilogram meter per second squared. And I don't necessarily even have to know that that's called a Newton. So you don't necessarily need the SI name, although it'll often have a name. All you really need to know is how it's built up of these other more fundamental things. And that powerful thinking can even go backwards. Sometimes if you know that the unit very well, you can actually make a fairly good guess as to what that thing is. So for example, a Pascal, which is a unit of pressure, if I tell you that that's one Newton per meter squared, which we could turn again into kilograms and seconds and meters if we wanted to. But if you knew that, then you can kind of tell that it's a force per unit area. So Newton is an amount of force and meters squared is an amount of area. And indeed, pressure is how much force per unit area something has. So how many of the really fundamental quantities are there? Well, it turns out that nearly all of the SI units can be based off only four. The kilogram, the second, the meter, and the Coulomb. So there's this stuff called mass. There's this stuff called charge. Coulomb is a unit of charge. There's this stuff called distance and stuff called time. And out of those four, you can build nearly all the others. The only exceptions are the candler, which has something about how the standard human eye responds to light and the Kelvin, which has something to do with temperature. So these four fundamental quantities and nearly all physical quantities are related to them. Because the SI units have to be designed to be easy to measure as well as standard for everybody, they don't actually contain these four fundamental quantities. They do contain the meter and the kilogram and the second. But the SI base units don't use the Coulomb. They use the ampere, which is the unit of electric current. That's Coulomb per second. That's the one that's defined because that's easier to measure. It turns out there's also in the base units, the Kelvin, mole, and the candler. And those base units can be combined to produce all the other SI units.