Logic Riddles

Question: I'm priceless, and cannot be bought, Some seek my purpose, Others destroy me, Those who steal me gain no more, I never last forever, but can a life-time. What am I?

Question: Why do cows wear bells?

Question: How do you tell if a snake is venomous or not?

Question: What grows when you take from it?

Question: A man wanted to enter and exclusive club, but did not know the password to get in. He waited by the door and listened. The doorman said "Twelve" and the other man said "Six" and was let in. A second member came up and the doorman said "Six" to which the member replied "Three" and was let in. The man went up and the doorman said to him "Ten" the doorman said "Five" but was not let in. Why?