What has hands, but can’t clap?

Replace the question mark with the correct number in below Picture:

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?

Why do you find that most of the buildings are deprived of the 13th floor?

Forwards I am heavy, backward I am not. What am I?

The more it dries, the wetter it becomes. Can you tell what it is?

You have a three-gallon and a five-gallon measuring device. You wish to measure out four gallons.

There are 2 cops parked along a one-way street looking for traffic violations. They spot a taxi driver going in the wrong direction, yet they do nothing.

Why?

Pronounced as one letter,
And written with three,
Two letters there are,
And two only in me.
I am double, I am single,
I am black blue and grey,
I am read from both ends,
And the same either way.

our enemy challenges you to play Russian Roulette with a 6-cylinder pistol (meaning it has room for 6 bullets). He puts 2 bullets into the gun in consecutive slots, and leaves the next four slots blank. He spins the barrel and hands you the gun. You point the gun at yourself and pull the trigger. It doesn't go off. Your enemy tells you that you need to pull the trigger one more time, and that you can choose to either spin the barrel at random, or not, before pulling the trigger again. Spinning the barrel will position the barrel in a random position.

Assuming you'd like to live, should you spin the barrel or not before pulling the trigger again?