How do you explain decimeters and centimeters to a 7 year old?

Answer 1

I'd tell a story. This is what I've told my neices, nephews, and cousins - starting in kindergarten. Did you know that most of the world doesn't measure things like we do? It's because of a very naughty king who got to have his way all the time. He stuck out his thumb and had his assistant measure from his nose to his thumb and called it a yard. That was okay for about 50 years until there was a new King. The next king was really a small guy. His arm was a lot shorter. Suddenly the real length of a yard changed. Now people owned a LOT more land and their taxes went up. Everyone was mad, except the king who got a lot more money. So the really smart people decided that the world shouldn't use the king's arm anymore, and when they showed him that the royal tax income would go down if a long arm king got to be king.. the king relented and let the smart people figure out a better way to measure stuff.

Those brainiacs invented the decimal system. But the next king was short armed too, and he decided to not publish the news about the new way of doing thing and not to push the new measuring system. When things go on and on for a long time, people just accept ideas the way are and don't question them. So because of the short armed kings, everything we measure now is just a mess and way harder than it has to be. Scientist finally got tired of all the extra math and gave up on the silly way the king decreed and so scientists in our country and most of the other countries in the world do it an easier way. It's called the decimal system. Here is how most people in the world that didn't follow those silly kings actually measure things.

How many fingers do you have? toes? (hopefully the kid has 10 of each)

Great. how many ones are in the number 10? (use your pointer to count to 10)

10! Wow! 10 is really easy to count right? well, that's what most people in the world that didn't have to follow the king used too! They naturally groups things into 10's. So, if you include all your toes and fingers how many batches of 10 do you have? and, if you include all my fingers and toes too, how many batches of 10s do we have together?

Answer 2

Decimeters are 10 times longer than centimeters. If you think of a centimeter like a small step, a decimeter would be like taking 10 of those steps in a row. So, a decimeter is a bit bigger than a centimeter.