 Units. In science, units are king. When you make a measurement, it's meaningless unless you know what unit it was measured in. This is one of the key differences between maths and science. For much of the time in maths, you're interested in numbers just for their own sake. If you mention the number 10, then that means exactly 10, no more, no less. But in science, if you say you've measured something to be 10, you'll immediately be met with the response 10 what? Well, you might say I'm holding a stopwatch and I was timing this cyclist so obviously it seconds. Really? Obviously? What if it were minutes? Or the cyclist was doing an endurance event and it were hours? Or maybe you're timing the cyclist's heartbeat or how long it takes the bike wheel to complete 1 360th of a revolution? And how do I know you're not measuring in shakes or jiffies or micro fortnights? Well, okay, by looking at the stopwatch. But you see my point. If you don't have the unit, then the number itself gives you no useful information. So our watchword in science is no naked numbers. Make your numbers decent by giving them units. In chemistry, we use metric units. The modern form of the metric system is known as the international system of units, which has seven base units. And from these, the units of all other quantities can be derived. Two of these units are not so common in chemistry. There's amperes, the unit of electrical current, although you may encounter this in electrochemistry, and candelas, the measurement of the intensity of light. The other five though will turn up frequently. We have kilograms for mass, meters for length, seconds for time, Kelvin, degrees Kelvin for temperature, and moles for quantities of atoms and molecules. Kilograms, meters and seconds you'll already be familiar with. Degrees Kelvin work much like degrees Celsius, and we'll look at this in another video. Moles are the unit which is central for chemists. We'll be using this unit constantly as we learn how to calculate chemical quantities.