What am I riddles with simple answers

While I am in air, I am not in oxygen. I am also in water, but not in hydrogen. I am necessary in all animals, but you won't find me in the zoo. Look in all brains and you'll find me there too. What am I?

I have keys but no locks. I have a space but no room. You can enter, but can't go outside. What am I?

I have lot of memories, but I own nothing. What am I?

I hiss. I can be poisonous. I am long. I have a forked tongue. What am I?

I have no life, but I can die, what am I?

I have seas without water. I have forests without wood. I have deserts without sand. I have houses with no brick. What am I?

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

What has one head, one foot and four legs?

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 visit you every night even if you don't call me. I'm lost every day. What Am I?