1. Fill the big barrel first, then fill the small barrel with the water in the big barrel, and pour out the water in the small barrel. At this time, there are 2 kilograms of water in the big bucket; 2. Pour the remaining water in the big bucket into the small bucket again and fill the big bucket; 3. Fill the small bucket with the water in the big bucket and pour out the water in the small bucket. At this time, there are 4 kilograms of water in the big bucket; 4. Pour the remaining water in the big bucket into the small bucket again and fill the big bucket; 5. Fill the small bucket with the water in the big bucket and pour out the water in the small bucket. At this time, there are 6 kilograms of water in the big bucket; 6. Pour the remaining water in the big bucket into the small bucket. At this time, there is 1 kg of water in the big bucket.