Difficult Riddles

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

What's the greatest worldwide use of cowhide?

Suppose you want to send in the mail a valuable object to a friend. You have a box which is big enough to hold the object. The box has a locking ring which is large enough to have a lock attached and you have several locks with keys. However, your friend does not have the key to any lock that you have. You cannot send the key in an unlocked box since it may be stolen or copied. How do you send the valuable object, locked, to your friend - so it may be opened by your friend?

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.

Mrs. Jones was very proud of her apple tree. One autumn, after harvesting her apples, she called her three sons together. "Here are 150 apples," she said. "I want you to take them to the market tomorrow and sell them for me." She gave Paul 15 apples, Nick 50, and Ben 85. "Your job," added Mrs. Jones, "is to sell the apples in such a way that each of you brings home the same amount of money." How do they do it?

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?