(Originally posted at http://joelnoche.multiply.com/journal/item/109/Negative-numbers on August 20, 2012 5:57 AM)
When my daughter just turned four years old, she presented me with a piece of cardboard where she wrote some arithmetic expressions that she wanted me to evaluate (shown at the left).
I told her that I was afraid her questions were too advanced, and that I was worried that she wouldn’t be able to understand my explanations. I stated to explain why subtracting a greater number from a lesser number is not so easy for someone her age to understand. She then realized that I was reading her questions upside down. (See the comment below for the continuation of my story.)