The young woman walks up to a stall piled high with coconuts. Behind it stands a young boy of around 12 years of age, who is taking care of the stall while his parents have stepped away for a moment. It’s hot and there is a lot of noisy activ-ity in the market, one of several in the Brazilian city of Recife.“How much is one coconut?” the woman asks. “Thirty-five,” the boy replies with a smile. “I’d like ten. How much is that?” The boy pauses for a moment before replying. Thinking out loud, he says: “Three will be 105; with three more, that will be 210. (Pause) I need four more. That is . . . (pause) 315 . . . I think it is 350.”Though the boy gets the answer right, the woman can’t help but wonder why he did not use the simple rule that to multiply by 10 you just add a 0, so ten coconuts at 35 Cruzeiros each will cost Cr$350. A short while later, at another stall, again staffed by a young boy, this one about 14 years old, the woman makes a purchase that requires the child to sub-tract Cr$75 from Cr$243. The boy calculates out loud: “You just give me the two hundred. I’ll give you twenty-five back. Plus the forty-three that you have, the hundred and forty-three, that’s one hundred and sixty-eight.”If you were the shopper, faced with a young child at a stall in a noisy, busy South American street market who calculated your change in that fashion, you might suspect that the young salesman was trying to pull a fast one. In fact, his answer is perfectly correct. In a moment I’ll examine what he is doing.