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

I am a type of vehicle that has two wheels and is powered by humans. What am I?
Hint 1: You can ride me for exercise.
Hint 2: I am environment-friendly.

You find yourself in a strange place guarded by two guards.One of the guard always say truth while other always lies.You don't know the identity of the two.You can ask only one question to go out from there. What should you ask?

In two decks of cards, what is the least amount of cards you must take to be *guaranteed* at least one four-of-a-kind?

Two in a corner, one in a room, zero in a house, but one in a shelter. What am I?

A Doctor always sneezes before the rain. Doctor sneezes, Will it rain?

What can run but never walk, have a mouth that never speaks, have a head that never weeps, and have a bed but never sleeps?

John is asked to stand behind Jacob and Jacob is asked to stand behind John. Both of them feel confused about it and don't know what to do?

But it is quite possible, do you know how?

Can you find out which number multiplied by itself will give the output as 12345678987654321?

0 0 0 = 6
1 1 1 = 6
2 2 2 = 6
3 3 3 = 6
4 4 4 = 6
5 5 5 = 6
6 6 6 = 6
7 7 7 = 6
8 8 8 = 6
9 9 9 = 6

You can use any mathematical symbols in the space provided to make all above algebraic expressions true.