A family of elephants has wandered close to a village.  They're beautiful, but they eat a lot and can be dangerous.  They couldn't stay.  Some scientists shot them with darts that made them sleepy.  But these elephants won't stay sleepy for long.  It's your job to move them safely to a place where there's plenty of food and they won't harm any humans.  Here are a few questions you should ask: How many trucks will you need?  How big are these elephants?  How far do you need to carry them?  Will you be able to do it all before the elephants become fully awake again?


First, it would help to know how many elephants need to be moved.  Let's see. . . one, two, three, four, five, six, seven elephants.  That's a lot of elephants.  A unit is what we use to measure the number of something.  It can be anything from seven apples to seven ants to seven eggs to seven elephants to seven feet to seven seconds.  It can also be used to talk about how far away something is, but we will get to that in a minute.  Right now we will need seven more units of something else.  What do you think that is?  You're right.  Trucks. 


Units help us know what we're talking about.  I could tell you I have seven of something and you would have no idea what I was talking about.  We use the word expressed to tell what kind of units you are counting.  We could say seven units expressed in elephants.  Now that you have that down, we can get to saving these elephants.

Do you count seven elephants? I count seven elephants.


We have seven trucks for seven elephants.  But what if one of the trucks is too small?  We had better do some measuring.  A meter is about three feet long.  That is about the length from the tip of your nose to the tips of your finger, or a tiger's tail.  You can use meters to measure anything from the size of a bus to the size of a playground to the size of an elephant.  Let's see, the biggest elephant is. . . wow, 3 meters tall, 2.5 meters long, and 2 meters wide.  As long as we get trucks that can hold that, we're ready to rumble.

Looks like we're going to need more trucks.


Great, now we have got all of our sleepy elephants loaded onto the trucks.  We need to get them far away from this town so they will not come back and eat all the plants again.  In other words, we are not just going to drive them a few meters away.  A kilometer is a thousand meters.  We use it when talking about long distances, like driving or flying.  Elephants will walk for days and weeks if they really want to get somewhere, so we are going to take them to a place that's 500 kilometers away.  We don't want the elephants to wake up before we get there, so our driver will need to drive fast.


The elephants are in a safe place now.  You expressed numbers in units to find out how many trucks we needed.  Then you measured the elephants in meters to make sure they fit in the truck.  Finally, you measured the number of kilometers they needed to go before they woke up.  The trip went quickly, so everyone is safe and happy.  Without these measurements, you might have had trucks that were too small, and you might not have given the driver enough time to get the elephants safely away.  Everyone likes a happy elephant.


References:

Math Is Fun.  "Metric Length"  Math Is Fun, 2010.  <http://www.mathsisfun.com/measure/metric-length.html>