Good Riddles

Question:

You have a large number of friends coming over and they all get thirsty. Your first friend asks for 1/2 a cup of water. Your second friend asks for 1/4 a cup of water. Your third friend asks for 1/8 a cup of water, etc.

How many cups of water do you need to serve your friends?

Question: What is a word made up of 4 letters, yet is also made up of 3. Sometimes is written with 9 letters, and then with 4. Rarely consists of 6, and never is written with 5.

Question: There are several different kinds, but the one you pick doesn't do its job.

What is it?

Question: It takes ten men ten hours to build a certain wall. How long does it take five men to build the same wall?

Question: A magician was boasting one day at how long he could hold his breath under water. His record was 6 minutes. A kid that was listening said, "that's nothing, I can stay under water for 10 minutes using no types of equipment or air pockets!" The magician told the kid if he could do that, he'd give him $10,000. The kid did it and won the money. Can you figure out how?