If eleven plus two equals one, what does nine plus five equal?

The person who makes it has no need for it. He who purchases it does not use it. The person who does use it does not know they are. What is it?

When I go in I might cause pain. I cause you to spit and ask you not to swallow. I can fill your hole. What am I?

There are people and strange monkeys on this island, and you can not tell who is who (Edit: until you understand what they said - see below). They speak either only the truth or only lies.
Who are the following two guys?
A: B is a lying monkey. I am human.
B: A is telling the truth.

Do you know of a place where the wind blows south and then suddenly shifts direction towards the north?

What goes up but never comes down?

There is a number which when you multiply by 3 and subtract 2 from the result, then the resulting number is the reverse of the actual number.

What is the smallest number that stands true on the statement?

I have an eye but still cannot see.

Who am I?

If you drop me I’m sure to crack, but give me a smile and I’ll always smile back. What am I?

The teacher told the student that if he told a lie then he will be expelled from school and if he told the truth then he still is expelled from school.

What can a student say to prevent his being expelled from school?