Edward de Bono introduced the term Lateral Thinking. Nowadays it is used quite commonly. Lateral thinking requires an open mind and you need to be very aware of hidden assumptions. It’s an important skill in finding different solutions than the obvious ones to all kinds of questions. Einstein even took it a level deeper:

The significant problems we have cannot be solved at the same level of thinking with which we created them.
  - Albert Einstein (1879 - 1955)

Riddles are a good way to train your lateral thinking skills. Here’s a list of 21 riddles:

1. The strongest chains will not bind it
   Ditch and rampart will not slow it down
   A thousand soldiers cannot beat it
   It can knock down trees with a single push
    
2. The one who made it
   Didn’t want it
   The one who bought it
   Didn’t need it
   The one who used it
   Never saw it
    
3. Buckets, barrels, baskets, cans;
   What must you fill with empty hands?
    
4. Five pieces of coal, a carrot and a scarf are lying on a hill near a
   remote house. Nobody put them on the lawn but there is a perfectly
   logical reason why they should be there. What is it?
    
5. Every creature in the world has seen it
   But to their dying day they’ll never see
   The same one again
    
6. Look in my face
   I am somebody
   Look at my back
   I am nobody
    
7. A man leaves home and makes 3 left turns.
   When he returns home he is met by two men
   one of whom is wearing a mask. Who are they?
    
8. We love it more than life
   We fear it more than death
   The wealthy want for it
   The poor have it in plenty
    
9. The light one breaks but never fails
   His brother fails but never breaks
    
10. Which four letter sport begins with a ‘T’ ?
     
11. Assume there are approximately 6,000,000,000 (6 billion) people on
    Earth. What would you estimate to be the result, if you multiply
    together the number of fingers on every person’s left-hands? (For the
    purposes of this exercise, thumbs count as fingers, for five fingers
    per hand.) If you cannot estimate the number then try to gues how long
    the number would be.

12. An archeologist proudly told that he had found four silver coins which, according to the inscription "649 B.C.", should now be 2720 years old. The press considered him a fraud and a dreamer. Why?
     
13. He got it in the woods
    And brought it home in his hand
    Because he couldn’t find it.
    The more he looked for it
    The more he felt it. When he
    finally found it he threw it away.
     
14. This wondrous thing, though not
    An herb, can help comfort the weak
    And the dying. It can even be used to
    Rally the troops, or make one start
    Laughing or crying
     
15. A New York city hairdresser recently said that he would rather cut the hair of three Canadians than one New Yorker. Why?
     
16. This sparkling globe
    Can float on water
    And weighs no more
    Than a feather
    Yet despite its weight
    Ten giants could
    Never pick it up
     
17. How could a baby fall out of a twenty-story building onto the ground and live?
     
18. Whoever has it is angry
    Whoever loses it is even angrier
    Whoever wins it has it no more
     
19. This engulfing thing
    Is strange indeed
    The greater it grows
    The less you see
     
20. It can pierce the best armor
    And make swords crumble with a rub
    Yet for all its power
    It can’t harm a club
     
21. Today he is there to trip you up
    And he will torture you tomorrow
    Yet he is also there to ease the pain
    When you are lost in grief and sorrow
     

Please post your answers in the comments. I’m sure you’ll be able to solve all of them

Numbers 7, 10 and 12 are from Creative Puzzles
Numbers 4, 11 and 15 are from Lateral Thinking Problems
Visit their sites for a lot more riddles.