 Of course, it's not enough simply to have kilograms, meters and seconds. Sometimes we want to measure things that are very light or very long or very fast. So then we change the size of the unit by adding a prefix. Some of these prefixes you know already. For instance, kilogram has one built in. Kilo means 1000. So one kilogram is equivalent to 1000 grams. The prefixes you most need to be familiar with are these. Kilo, desi, centi, milli, micro and nano. You can always look the conversions up but if you know them off by heart it speeds things up when you're plowing through a long calculation. So let's use meters as an example. One kilo meter is equivalent to 1000 meters because kilo means 1000. Desi means a tenth. A decimeter is equivalent to a tenth of a meter or 0.1 meters. And this means the same as saying that 10 decimeters is equivalent to 1 meter. Centi means one one hundredth. One centimeter is equivalent to 0.01 meters or we can say that a hundred centimeters are in a meter. Millie means one one thousandth. One millimeter is equivalent to 0.001 meters, one times ten to the minus three meters. Or we can say that there are a thousand millimeters in a meter. Micro means a millionth. One micrometer equals one times ten to the minus six meters. Or we can say that there are one million micrometers in a meter. Micrometers are also sometimes called microns. And finally, nano means one billionth. So a nanometer is one times ten to the minus nine meters. Or we can say that there are one times ten to the nine nanometers in a meter. Make sure you're familiar with these prefixes. You'll encounter them frequently.