Fill the small bucket with water first, then pour it into the big bucket, then fill the small bucket with water, then pour it into the big bucket, then fill the small bucket with water, and then pour it into the big bucket. When pouring water for the last time, because the big bucket can only hold 1 1kg of water, then fill it with more 1kg, and then pour out all the water in the big bucket, so as to add more water to the small bucket.