>>14
Don't say all that at once. Just say "I'm planning on becoming a physicist". Then they'll probably respond with something like "so you're going to be the one in charge of [something] working right" or "what type of stuff will you be doing". Then elaborate on what you do. This way you aren't pushing too much information on the person all at once and you come off a lot less pretentious. Also, when you do elaborate, start off by giving examples "Well I'll be working on the physics of what happens under high density conditions for example [something that the listener is probably familiar with]" or "Well basically working on a number of theories that deal with the possibility of time travel".
>>12
Only morons learn math and science to try and appear smarter than others. True lovers of math and science can spot fakes like you from a mile away.
I don't try to explain things to people unless it comes up in a conversation and they're curious about it, or unless they themselves ask about it. When they do, I generally start off by telling them how hard of a concept I thought it was to understand. Then I think of an analogy (or in math an application) and elaborate on that. An explanation of something well generally go something like this:
"Ok, so we know 'A' about 'X' ~{pause}~ and we also know 'B' about 'X' ~{pause}~ and because of 'A' and 'B' we can say that we know 'C' about 'X'; ~{slightly longer pause}~ but what if we want to find out 'D' about 'X' ~{slightly longer pause}~. Well, if we think about it this way ~{draw large picture or chart or whatever and label things as I mention and explain them}~, then we can see that 'D' is actually just [insert property] (minus/plus/times/etc...) [insert property]. ~{pause}~ So what does that mean? ~{pause}~. Well when 'A' and 'B' are [insert property] then 'D' is going to be [insert property] and when 'A' and 'B' are [insert property] then 'D' is going to be [insert property]."
This way the person listening can take advantage of the pauses to ask questions, just think about things, or whathaveyou. The material should always be easy to follow, but talked about in such a way that it all feels like common sense to the listener. If the listener complements the explanation I'll try to give credit where it's due ("yea, I thought it was clever too. Some guy was talking about it on the internet" or something of the sort). This way they don't feel intimidated and are more likely to treat me as an equal.
Wow, this post really ran long. I guess it went off topic too. Sorry about that.
TL;DR: don't teach things in a manner that will intimidate the listener or you'll just look like a pretentious ass who doesn't know what he's talking about and you'll scare people away from the subject.