Divisibility Rule for 3 is Math not Magic

We all learn little "tricks" when it comes to math.  When I was in second grade, we learned subtraction by "borrowing" from one place over (not like we were going to give it back.)  Even with the better description of regrouping, many second graders are taught to memorize the "trick" for subtraction where you cross off and add a one to the tens place to the number on the right.    

While learning long division, 3rd graders at our local elementary school chant:  "Divide, Multiply, Subtract, Bring down"  and remember the steps by using (D- daddy, M-mommy, S-sister, B-brother.)  5th graders remember that to divide fractions, they flip the second fraction and multiply.  Unfortunately, all of this leads to students thinking that math is a bunch of tricks, that often don't make any sense. Students who memorize and compute well are "good" at math, when in reality, they are good at remembering patterns.  

I love to ask my students why they can do the "trick" to solve the math problem.  This makes some of them irritated (why are you making me do something other than the "math"), but some are curious.  These are the students who are budding mathematical thinkers, and are sometimes led to believe they aren't good at "math."

Here is a video explaining that in order to tell whether a number is divisible by 3, you can add the digits and divide the answer by three.  If it is without remainder, the whole number is divisible by 3.  Check it only if you are curious, because I would hate to irritate you!