Keyboard words are not in alphabetical order, Why?

A man has a barrel filled with oil that weighs 100 pounds, and then he puts something into it. Now the barrel weighs less than 100 pounds. What did he put in the barrel?

You are trapped in a room with two doors. One leads to certain death and the other leads to freedom. You don't know which is which.
There are two robots guarding the doors. They will let you choose one door but upon doing so you must go through it.
You can, however, ask one robot one question. The problem is one robot always tells the truth, the other always lies and you don't know which is which.
What is the question you ask?

What's a single-digit number with no value?

What is more useful when it is broken?

Look at this sequence from top to bottom. What is the next number in the sequence?
1
11
21
1211
111221
312211

Replace the question mark with the number below given a simple math problem.

I never stop running even when I standstill. If I am formed by joining two identical bodies together, can you guess who I am?

If two's company, and three's a crowd, then whats four and five ?

Count the number of balls in picture below: