Metric Length

Measuring how long things are, how tall they are, or how far apart they might be are all examples of length measurements. 
These are the most common measurements::
 Millimeters
 Centimeters
 Meters
 Kilometers


The smallest units of length are called millimeters.
A millimeter is about the thickness of a plastic id card (or credit card).
Or about the thickness of 10 sheets of paper on top of each other.
This is a very small measurement! 

Centimeters
When you have something that is 10 millimeters, it can be said that it is 1 centimeter.
1 centimeter = 10 millimeters
A fingernail is about one centimeter wide. 
You might use centimeters to measure how tall you are, or how wide a table is, but you would not use it to measure the length of football field. In order to do that, you would switch to meters.
Meters

A meter is equal to 100 centimeters.
1 meter = 100 centimeters
The length of this guitar is about 1 meter
Meters might be used to measure the length of a house, or the size of a playground. 
Kilometers
When you need to get from one place to another, you measure that distance using kilometers. A kilometer is equal to 1,000 meters.
The distance from one city to another or how far a plane travels would be measured using kilometers. 

Final thoughts about measuring length:
1 centimeter = 10 millimeters
1 meter = 100 centimeters
1 kilometer = 1000 meters
From Very Small to Very Large
Here is an illustration of smallest sizes to largest sizes, using metric numbers like milli and kilo (just add the word "meter" after them, so you get "millimeter", "kilometer", etc).
Interesting facts:
 Quarks are very very small
 Molecules are around the billionths of a meter in size. That is 0.000000001 meters. Some molecules are smaller and some bigger, though.
 People are a little over a meter tall,
 Mountains are kilometers in size.
 The Earth is megameters in size (a megameter is a thousand kilometers, and the Earth's Diameter is actually 12,000 km)
 A Light Year is about 10 petameters in size (a petameter is 1,000,000,000,000,000 meters, which is a 1 followed by 15 zeros)
 The Milky Way is about 1 zetameter across (1,000,000,000,000,000,000,000 meters, which is a 1 followed by 21 zeros)
 The Universe is very very big
The numbers use scientific notation to show how big the value is.
Example: 10^{6} is 10 used in a multiplication 6 times, which is a 1 followed by 6 zeros: 1,000,000.
It is also called a million.
The prefix is mega, so a megameter is a million meters.
Example: 10^{9} is a 1 moved nine places the other side of the decimal: 0.000 000 001
It is also called a billionth.
The prefix is nano, so a nanometer is a billionth of a meter.
You can also use Metric Numbers with other measures like seconds, grams and so on.
More Examples
A centimeter (cm) is about:
 about as long as a staple
 the width of a highlighter
 the diameter of a belly button
 the width of 5 CD's stacked on top of each other
 the thickness of a notepad.
 the radius (half the diameter) of a US penny
A meter is about:
 a little more than a yard (1 yard is exactly 0.9144 meters)
 the width of a doorway (most doorwaya are about 0.8 to 0.9 m)
 half the length of a bed
 the width of a large fridge
 the height of a countertop
 four rungs up a ladder
 five steps up a staircase
 the depth of the shallow end of a swimming pool
 the width of a dining table
 the height of a 5 year old
 shoulder to opposite wrist of an adult
 outstreched arms of a child
 waist high on an adult
 a long stride
A kilometer (km) is about:
 a little over half a mile
 a quarter of the average depth of the ocean
 how far you walk in 12 minutes
