The following question comes to us from fifth grader Savannah, from Ruston. "How do you convert units with exponents using the method 'king henry doesn't usually drink chocolate milk'?"

Hi Savannah. "King Henry Doesn't Usually Drink Chocolate Milk" is what we call a mnemonic; it's a phrase that's designed to help you remember something important. In this case, what we're trying to remember is the prefixes for metric units. The prefixes all begin with the same letters as the words of the mnemonic, as shown below, with an example unit:

King ⇒ kilo ⇒ kilometer
Henry ⇒ hecto ⇒ hectometer
Doesn't ⇒ deca ⇒ decameter
Usually ⇒ Unit without prefix ⇒ meter
Drink ⇒ deci ⇒ decimeter
Chocolate ⇒ centi ⇒ centimeter
Milk ⇒ milli ⇒ millimeter

So let's say I had 20 hectoliters. How would I convert that into milliliters? King Henry will help me remember the order of the units:

kilo ⇒ hecto ⇒ deca ⇒ unit (without prefix) ⇒ deci ⇒ centi ⇒ milli

If I want to go from hectoliters to milliliters, I'm moving 5 prefixes to the right, so I'm going to multiply by 10⁵. Thus, I have 20 x 10⁵ = 2000000 milliliters.

If I wanted to convert 5 centimeters to decameters, I am moving the opposite direction through the units, and I'm moving 3 prefixes. So this time I divide (because I'm moving in the other direction) by 10³. 5/10³ = 0.005 decameters.

One problem with this mnemonic is that both deca and deci begin with D, so you still have to remember which word goes with which prefix. I would try to remember that both "drink" and "deci" have the letter i in them, and that may help you out.

Good luck with your unit conversions!