Difficult Riddles

What's the greatest worldwide use of cowhide?

What is it the more you take, the more you leave behind?

I come in darkness, but fill the mind with light. I bring enlightenment to some, while gripping others in the hand of fear. With me it can be a journey of inexplicable joy and sorrow. What I will show you will often be unreachable. Journey with me and what you see may haunt you. Journey with me and you may never want to return home. Journey with me and you will never know when it will end. What am I?

In "The Hobbit" by J. R. R. Tolkien, there are several hard riddles that Gollum and Bilbo ask each other.

Alive without breath,
As cold as death;
Never thirsty, ever drinking,
All in mail never clinking.

The rungs of a 10 foot ladder attached to a ship are 1 foot apart. If the water is rising at the rate of one foot an hour, how long will it take until the water covers over the ladder?

When you stop and look, you can always see me. If you try to touch you cannot feel me. I cannot move, but as you near me, I will move away from you. What am I?

Two words, my answer is only two words. To keep me, you must give me.

There are three crates, one with apples, one with oranges, and one with both apples and oranges mixed. Each crate is closed and labeled with one of three labels: Apples, Oranges, or Apples and Oranges. The label maker broke and labeled all of the crates incorrectly. How could you pick just one fruit from one crate to figure out what’s in each crate?

I'm so fast you can't see me, Though everyone sees straight through me, I don't stop until the day you die. What am I?

Which clock works best, the one that loses a minute a day, or the one that doesn't work at all?