Good 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: What has 4 fingers and a thumb, but is not living?

Question: I'm light as a feather, yet the strongest man can't hold me for more than 5 minutes. What am I?

Question: What belongs to you but others use it more than you do?

Question: Why is a raven like a writing desk?