Funny Riddles

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: What is the easiest way to poke a balloon without popping it?

Question: Draw, fire, or fill me;
I'm still empty.

What am I?