Riddle: You have two buckets: 5L and 3L. Assuming that you have unlimited chances of refilling the bucket, how do you obtain 4L of water? You cannot use any other container. You have a sink to drain water, but when you refill or drain you can only do full amount. You cannot drain half the water or only fill a little water.
Answer: 1. Fill the 5L bucket and use it to fill the 3L bucket. You now have a full 3L bucket and 2L of water in the other. 2. Pour the 2L of water into the 3L bucket. Now you have a 3L bucket with 2L water in, and an empty 5L bucket. 3. Fill the 5L bucket and fill the 3L bucket. Because the 3L bucket has only 1L space left from step 2, it can only hold 1 more litre. From this, you get 4L of water left in the 5L bucket.