Turns out we have a smart dog.
I had heard that you can train a dog to ring a bell to be let out. There's nowhere to put a doggy door in our house, so I rigged up a bunch of bells on a string and hung it from the back door. I intended to get around to teaching him, someday, that if he bats at it, he can go out. Once or twice when I was letting him out, I'd give the bells a shake, but I didn't do it with any consistency. Of course when you open the door, the bells do sort of rattle against the door somewhat.
A couple of days ago I was on the other end of the house, the girls were upstairs, and I hear a persistent shaking of the bells. I came to the back door and there is the dog, sitting at the back door, watching the door. He looks at me as I come up, then he looks at the door. I let him out.
This morning as we were having breakfast he got up and went over to the door. I hadn't let him out yet, and was kind of wondering what he'd do. Thing 2 noticed him by the door and went over to pet him. He reached up with his paw and batted at the bells, then looked at the door. She let him out.
So this evening I was folding clothes, and he noses the bells, and sits by the door. I get up and let him out. After several minutes I hear him barking (it's approaching midnight and I can't have him waking the neighbors) and I call him back in. He putters around me, I wave him off my folded laundry piles, he lays down for a while. Then it seems he gets bored, and goes to the back door and noses at the bells and sits and looks at me.
He's gone and taught himself to ring the bells to be let out. Now I'm at his beck and call to hop up and let him out whenever he rings. He's too smart for my own good.