Some time ago, my 4 year old son proudly showed me how he could make 6 adding 3+3, and 2+2+2. I showed him we could do 5+1 and 4+2 too. Then he realized we could make 1+2+3 and 1+1+4! I was so excited I decided to push him a little further and showed him we could make 6+0=6! His smile faded from his...