My house has a doggie door that goes from Youngest Son's room out to the garage. Youngest loves it, and use it most of the time to go from the house to the garage.
The door is 13 1/2 inches tall, and 9 1/1 inches wide. I assumed that the only member of the household who could fit through the door was Youngest.
Today, I discovered that my assumption was incorrect. My 98 pound Great Pyrenees can fit through the door. And Middle Son, who is 5'2" and weighs 125 pounds, can also fit through the door.
I found Willow out in the garage this afternoon, and I find Middle Son there after dinner. In both instances, the chain was still on the door, so the only way they could have gotten out through the doggie door.
I know what my next household project is.