Pour the water into a bucket that can hold 3 kilograms of water, and then there are 2 kilograms left.

Empty the bucket that can hold 3 kilograms of water, pour the remaining 2 kilograms of water into the bucket that can hold 3 kilograms of water,

Then fill the bucket that can hold 5 kilograms of water, and one * * * is 7 kilograms.