Coupon! Math in the Moment

I love the way Dan Meyer does such a wonderful job of recognizing math problems in the moment. So I tried to catch a moment myself today. During lunch when I was looking through the local grocery ads, ds8 saw a coupon for candy bars, 5 for $2. He remarked, "That's not a good deal! You can get a candy bar for like $.68!" I asked him how much one candy bar would cost if they are 5 for $2 with the coupon. He didn't know. We've only done a tiny bit of basic division, and certainly not division with money, but I decided that he was probably up to this task. So I cut out the ad and told him to use pictures, words, and/or numbers to figure out how much one candy bar would cost.

I knew this would cause a bit of disequilibrium. But I also knew that the right amount of disquilibrium = a great environment for new learning. After thinking a bit, he wrote out a long problem of 10+10 until he'd written 20 tens. He then made 5 boxes and drew curved lines connecting a 10 to each box. In the middle of this he stopped and said, "I know! They cost $.40!" I asked how he knew. He said that there are 20 tens in $2. And 20 divided by 5 equals 40. (Since he seemed to be on the right track and was obviously thinking in decimals, I didn't interrupt.) Then, after a while, he grinned and said, "We should get 4 candy bars for $1.60. Then [brother] and I can each have two so it's fair."

When do you successfully use math in the moment?