You put them on various household items such as a door or refrigerator so if your mom didn't open the fridge by 10 o'clock in the morning we would call you or send you a text message.

I'm the classic absent-minded professor: I'm very focused on something, and meanwhile, I've left the refrigerator door open for hours.

On a very local scale, a refrigerator is the center of the universe. On the inside is food essential to life, and on the outside of the door is a summary of the life events of the household.

I have an open-door policy.

An open door invites callers.

The door is now open for discussion,