My son has been fascinated by arithmetic lately, doing small sums and counting them out on his fingers — two plus two is four, three plus three is six, and so on. This morning he independently invented subtraction: “Three fingers, take away two, you have, one finger!”
I helped him do a few subtractions, but he was somewhat confused about zero, so we talked about that a bit, with examples: “If you have one banana, and you eat it, how many do you have left?”
“Zero!”
“And if you have two bananas, and you eat two, what do you have?”
He replied, very seriously: “If you eat two bananas, you have a tummy ache.”
Smart kid!