Good Riddles

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

Question: If you are smart and very good at math. Answer these 2 math riddles. 1. What is 0x29840847894657867478465875674896478496478567968. 2. 1x89498784893246798648732742974823.

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

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

Question: Half of those who read this riddle, I am your best friend.
I depend on you, but it's you I will defend.

What am I?