Why do people go to bed?

If an electric train is travelling south, then which way is the smoke going?

Replace each alphabet with the number (1-9) to make the below equation correct.

AB * C = DE - F = GH / I

They came out at night, without being called.
They lost at day, without being stolen.
Who are they?

A sea diver is a real show-off. He showed everyone that he can hold his breath underwater for 15 minutes.

I went to the diver and told him that I can be underwater for double the time i.e 30 minutes.
He responded that he will give me 100$ if I would be able to do it. I won 100$.

What did I do?

How will you measure 15 minutes using two hourglasses of 7 minutes and 11 minutes respectively?

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.

What is the fastest way to double your money?

Which letter of the alphabet has the most water?

Can you find out the number that should replace the question mark with a valid reason?