A mind is like a door.

Father William Devon (32) - The priest of the local Catholic church and known to be very liberal in his views, William has managed to increase youth attendance at church without alienating the older generation and is known for having a very open mind. As Pastor Michael James (49) of the Anglican church said about him, "An open mind is fine, but a mind is like a door: if you keep it open you might as well not have one." They don't get along.

But I'm fairly sure that that's not where I originally read the phrase.