Good Riddles

Question: I have keys but no lock, you can enter but not leave what am I?

Question: What is yours, but only others can use it?

Question: Why did the nudist not like going to London?

Question: No matter how terrible things get for the people of the Arctic, they will not eat a penguin.

Why not?

Question: I am a nice word composed of five letters, but the Romans would call me a numbers.

What am I?