What Am I Riddles

Question: Not chest or box is now discussed. Money can be held in it, but just as we test its metal, within it there is rust.

[What is it?]

Question: Enter one hole and leave three,
once you succeed I'm on thee?

What am I?

Question: Had by few but treasured by all,
I'm on the inside and outside and I make men fall.

What am I?

Question: You can rest on my pad without touching the keys.
I will sing all month long as long as you pay your fees.

What am I?

Question: I am a cave full of bones,
and the house of a worm.

What am I?