Which number should replace the question mark?

One of the funniest memories I have of the trials and tribulations of making the journey from childhood to adulthood was our annual summer vacation trek from Chicago to a cabin usually someplace on a lake in Wisconsin or Michigan.

Every year, it seems, we would get on a highway a few miles out of the city, and mom would wail, “Oh my goodness! I think left the iron on.” And almost every year we would turn around and go back. But as I recall, not once was it was ever plugged in. She often had the same fear that all our earthly possessions would disappear in a fire caused by her forgetfulness.

When I was about 14 years old, we were headed out of Chicago for Lake Geneva, Wisconsin and, sure enough, Mom gasped, “I just know I left the iron on.”

My father didn't say a word, just pulled over onto the shoulder of the road, got out, opened the trunk and handed her the iron.