How to have an unproductive conversation with a 5 year old child

Aidan: Mom, what are those metal things in the street?
Mom: What do you mean?
Aidan: Those round metal things.
Mom: Man-hole covers?
Aidan: Yes, I guess that's it.
Mom: That's how people get to the pipes to repair them.
Aidan: Do they leave the cover off when they are down there?
Mom: Yes, I think so.
Aidan: So they have light to work?
Mom: Well, they probably also take some lights down there.
Aidan: So maybe that would be a good way to get to somewhere like Califonia. Travel through all the pipes down there.
Mom: Um, if you went through the pipes you would probably end up at the city water supply.
Aidan: The what?
Mom: The lake where the city gets it water.
Aidan: Yeah (in his "duh" tone) so you could just get on a boat and sail across the sea and go back in those things (meaning pipes) and get to California.
Mom: This is ridiculous, we aren't going to talk about this anymore.
Aidan: I'm just trying to come up with a plan for us!
Mom: Okay, well on Sunday let's try that - we'll go down the manhole cover and go through the pipes to get to church!
Aidan: Nooooooooooooooo!