Easy simple riddles with simple answers

I'm tall when I'm young and I'm short when I'm old. What am I?

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

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?

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

What has a neck but no head?

When you have me, you feel like sharing me. But, if you do share me, you don't have me. What am i?

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

What can go up a chimney down but not down a chimney up?

I look flat, but I am deep, Hidden realms I shelter. Lives I take, but food I offer. At times I am beautiful. I can be calm, angry and turbulent. I have no heart, but offer pleasure as well as death. No man can own me, yet I encompass what all men must have.

What can be swallowed, but can also swallow you?