Multiplication
"Quickly, what is 1000 x 1000?"
"1,000,000."
"How did you arrive at the answer?"
"Simple. I just multiplied them together."
"You sure?"
"Of course!"
"Hmm... Now, what is 999 x 999?"
(Timed out thinking) bzzz---
"You didn't do the multiplication that time, did you?"
"Uhh... No, I didn't."
"What did you do?"
(Hang head in shame, low voice) "I counted zeroes. There are three zeroes in 1000. And three plus three is six."