Good Riddles

Question: Swallowing one is fine.
Swallowing two together is fine.
But Swallow them separately and all shall fall.

What are they?

Question: I fly and never rest. What am I?

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?