history: My dog just learned how to ring the bell on the doorknob when he needs to go out My dog just learned how to ring the bell on the doorknob when he needs to go out Whoa that is really cool. Did you train him to do that?
current: I put the bell on the door so it would ring when it opened and he got used to that, so now he associates the sound with having to go out. 
next: That is amazing. Must be a very smart dog.