AngeDog I think we should introduce your DS to our DD - the oxygen question is very much the sort of thing she would ask. In fact it sounds like a question she would ask having watched 'Here Comes the Science' by They Might Be Giants, which she adores. That's where the 'and if the sun was made of gas...?' questions come from. If you don't have it, I recommend it very much, but you need to be on your toes. We've been asked among other things what else is a plasma apart from the sun and what the names of all the elements are.
Your doctors/post office conversation is also very similar to the park/post office debate we had yesterday:
me: "We'll go to the Post Office on the way to the park, then we'll go to the park"
Kid: "because the Post Office sometimes closes early"
me: "Exactly"
Kid: "and if we don't go to the post office before the park?"
me: "well, daddy will have to carry this parcel all around the park"
Kid: "and the post office might be closed when we're finished"
me: "Yes, so we'll go there first I think"
Kid: "and if daddy had to carry the parcel around the park?"
Your DS sounds clever; the oxygen question at 2.7 is ace :) He's just after as much information as he can get to try and make sense of everything I think. I always assume the kid's questions are a result of her realising that we dont' always do exactly what we say we will, and trying to understand the reasons that might cause plans to change. I guess that way she learns more about each thing than if it just happened - e.g. if we had just gone to the post office and then the park she would have learned nothing because she's done that before. I'm not quite sure what she learned from the discussion we had yesterday, bit it will have been something, and she will weave it into her world view. I just love the way things we tell her come back to the surface months later having been analysed and digested.
That said, I can totally empathise with:
me (my plan is to get your cardigan on, get your shoes on and get in the car to collect mummy in the next five minutes)
kid (let's revisit the different shoe options, the occasions I've worn them all, their sizes, shoes I've had in the past and places I wore those, which shoes you can wear without socks, the car we used to have, the car we now have, the number of gears in the cars mummy and daddy owned before I was born, the route we will take, the places we might meet traffic, what mummy will say when we pick her up, what we will do when mummy gets home, things we need to check before we go out, what colour the door on our old house was ...)