Good Riddles

Question: What is the difference between a locomotive engineer and a teacher?

Question: What did the calculator say to the Math student?

Question: So a girl is scared of the dark, yet she always has the lights off in her room, why?

Question: What kind of coat can only be put on when wet?

Question: When you need me, you throw me away. But when you're done with me, you bring me back.

What am I?