Difficult simple riddles for teens with simple answers

I get whiter the dirtier I get. What am I?

It's red, blue, purple and green. No one can reach it, not even the queen.

What you cannot hold it but it is yours?

What has a foot on each side and one in the middle?

I build up castles. I tear down mountains. I make some men blind, I help others to see. What am I?

What letter is next in this sequence? M, A, M, J, J, A, S, O,__

Whoever makes it, tells it not. Whoever takes it, knows it not. Whoever knows it, wants it not.

I am something to make you think, I am something to make you know. I am something to which the answer you might never know. What am I?

Every time you stand up, you lose this. What is it?

When you have me more, you can see only less. What Am I?