A man took two buckets to fetch water. One barrel contains 3 kilograms of water, and the other barrel can hold 5 kilograms of water. How to get 4 kilograms of water now? Tell me why.

Fill a 3 kg bucket with water and pour it into a 5 kg bucket. Then take the second full bucket and pour it into a 5 kg bucket. When the 5 kg barrel is full, there is 1 kg left in the 3 kg barrel. Then pour out the water in the 5 kg barrel, pour the 1 kg water in the 3 kg barrel into the 5 kg barrel, then fill the 3 kg barrel and pour it into the 5 kg barrel, so that there will be 4 kg of water in the 5 kg barrel. (Because 6-5= 1 1+3=4)

Another method is to fill a 5 kg barrel with water and pour it into a 3 kg barrel. The 3 kg bucket is full, and there are 2 kg of water in the 5 kg bucket. Pour out the water in the 3kg barrel, then pour 2kg from the 5kg barrel into the 3kg barrel, then fill the 5kg barrel with water and pour it into the 3kg barrel. When the 3 kg bucket is full of water, there are 4 kg of water in the 5 kg bucket. (Because 5-3=2 2+ 1=3 5- 1=4)