Riddle #371

I’m the end of the colourful rainbow. What am I?

What am I?

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?

Forward I am heavy, but backward I am not. What am I?

I have 28 days in a month. Which month I am?

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

I dig out tiny caves, and store gold and silver in them. I also build bridges of silver and make crowns of gold. They are the smallest you could imagine. Sooner or later everybody needs my help, yet many people are afraid to let me help them. What am I?

I have four legs. I live in Africa. I am big and grey. I live in the river. What am I?

David's father has three sons : Snap, Crackle and _____ ?

I am taken from a mine, and shut up in a wooden case from which I’m never released, and yet I am used by many. What am I?

My best buddy always makes mistakes. But I would easily get rid of the mistakes. What Am I?