Difficult Riddles

What's the greatest worldwide use of cowhide?

A teacher writes six words on a board: “cat dog has max dim tag.” She gives three students, Albert, Bernard and Cheryl each a piece of paper with one letter from one of the words. Then she asks, “Albert, do you know the word?” Albert immediately replies yes. She asks, “Bernard, do you know the word?” He thinks for a moment and replies yes. Then she asks Cheryl the same question. She thinks and then replies yes. What is the word?

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?

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?

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?

Three men are lined up behind each other. The tallest man is in the back and can see the heads of the two in front of him; the middle man can see the one man in front of him; the man in front can’t see anyone. They are blindfolded and hats are placed on their heads, picked from three black hats and two white hats. The extra two hats are hidden and the blindfolds removed. The tallest man is asked if he knows what color hat he’s wearing; he doesn’t. The middle man is asked if he knows; he doesn’t. But the man in front, who can’t see anyone, says he knows. How does he know, and what color hat is he wearing?

You want to send a valuable object to a friend. You have a box which is more than large enough to contain the object. You have several locks with keys. The box has a locking ring which is more than large enough to have a lock attached. But your friend does not have the key to any lock that you have. How do you do it? Note that you cannot send a key in an unlocked box, since it might be copied.

NASA was considering sending canaries into space to study them under zero gravity. The project was scrapped when someone realized that in spite of having sufficient water supplies, they could die of dehydration within a few hours. Why?

As I went across the bridge, I met a man with a load of wood which was neither straight nor crooked. What kind of wood was it?