This happened to my daughter when she was 16 years old.
My daughter had gotten some wheels and her first job working at Subway. Well, you know how teenagers are, thinking they know everything. This was one of these discussions.
We were driving around town that day and she was talking about when she moved out and got an apartment for herself. I asked her if she'd still be working at Subway, or was she not going to think about going to college.
At this point in life, she was unsure about college. She didn't do great with math, and as it was a requirement for most any degree out there, she was having her hesitations.
So me, being the butthole mom that I am, decided to give her real life examples. I asked her how much she made there working at Subway.. which was minimum wage at the time. I think it was like $5 at the time. So I said okay, full time, 40 hours a week, and then after taxes, you are looking about having about $700 a month to live on.
She came back at that with 'but rent is like $500 for a small apartment here'. And this is where it got interesting.. I asked her if she was wanting to have electricity.. water.. did she want to eat.. gas money for her car.. insurance.. after adding up just some conservative figures I literally had her bawling in the front seat, crying "I'm going to have to live in a cardboard box!!!'.
And that is how you sometimes deal with know-it-all teenagers.. real world logic and some numbers to back it up! :)