Student: Can you do this problem for me?
Me: Nope, sorry--but I can help you understand it.
Student: But why don't you just do it for me?
Me: Because I won't be there for the rest of your life to solve your problems for you.
Student: But you could be!
Me: You'd have to pay me a lot of money to make me quit my job to be your personal mathematician.
Student: Not that much money. Teachers hardly make anything.