“Zero is a door.”

Lily was counting down by one hundred this morning on the way to school and, without me realizing it, she continued by zero to count into the negative numbers. When I realized what she was up to, I started asking her about negative numbers: what happens when you add to them? When you subtract from them? After a initial stumble or two, she quickly got hold of how things worked. I asked her how she figured it out so quickly, and she responded, “Zero is a door, daddy.”