Simple riddles

What has four legs, but can't walk?

I am a home of knowledge, both banal and profound. In grand halls and small homes I can be found. I am a home for things of many leaves, but my many residents are not living trees. What am I?

What happens when you throw a blue rock into the yellow sea?

What comes down and goes up again and again? What is that?

At night they come without being fetched. By day they are lost without being stolen. What are they?

What was the biggest island in the world before the discovery of Greenland by viking Erik the Red?

Pete is a worker on a large ship with hundreds of other men. He is constantly making mistakes, tying ropes the wrong way, steering the ship the wrong way, and making other blunders. One time he accidentally caused the ship to get stuck in a shallow shore, causing thousands of dollars in damage. However, when Steve, the captain of the ship, is asked about Pete, Steve says "I wish we had five men like Pete on this ship." Why would he say this?

I eat other animals. I have a big mouth. I am green. I live in the water. Who am I?

It can fill a room without occupying space. What is that?

What has a neck but no head?