I'm not sure my son was even 6 yet (he may have been five) when he learned about subtraction. We started by talking about things: to illustrate 4 minus 2, you start with four marbles and remove 2 of them, leaving 2. For a couple of weeks he would ask me to subtract numbers, then he moved on to asking me to ask him numbers (and writing down simply arithmetic problems to solve). Then, one day, out of the blue, he asked…
I had a similar experience when my oldest was still young. The conversation was a little flipped. We live in a cold climate and she had started taking note of the thermometer. I had asked her "what is 5 - 5?" > "zero!"; "what is that minus 5?" > "you can't do that!" > "What happens if it is 0 degrees out and it gets 5 degrees colder?" > * eyes open wide * "there are all the same numbers below zero!?"
Some time later, when she was learning algebra, I told her "0.999... == 1, can you tell me why?". I was expecting an algebra lesson to show her how that was true. She surprised me when, a second or two later, she looked up and said, "1/9 is 0.111... so 9/9 is 0.999... and 9/9 is 1". I was very proud.