You see a boat filled with people, yet there isn’t a single person on board. How is that possible?

How many sides does a circle have?

What is red and smells like blue paint?

John killed hundreds of people mercilessly and yet the police were not able to put her behind the bars. Why?

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.

Replace the question mark with the correct number, given the pair of numbers exhibits a similar relationship?

? : 3839 :: 11 : 1209

Decode this picture on T-Shirt and tell what it says ?

A woman lives in a Tall building thirty-six floors high and served by several elevators which stop at each floor going up and down. Each morning she leaves her apartment and goes to one of the elevators. Whichever one she takes is three times more likely to be going up than down. Why?

Below the four parts have been reorganised. The four partitions are exactly the same in both arrangements. Why is there a hole?

Place the numbers from 1 to 9 in the circles in a manner that each side of the triangle sums up to 17.