Step 1, divide 8 kilograms into 3 kilograms; step 2, pour the divided 3 kilograms into 5 kilograms; step 3, pour the remaining 5 kilograms in the 8 kilograms barrel into 3 kilograms, so that there are 2 kilograms in the 8 kilograms barrel, 3 kilograms in the 5 kilograms barrel and 3 kilograms in the 3 kilograms barrel; and step 4, pour the milk in the 3 kilograms barrel into the 5 kilograms barrel until the 5 kilograms barrel is full. Therefore, when it was filled up, it actually added 2kg, that is, there should be 1kg left in the 3kg bucket at this time. Step 5: Pour 5kg of milk into 8kg barrel, that is, 5+2 = 7kg in 8kg barrel at this time. Step 6: Pour 1kg from the 3kg barrel into the 5kg barrel. Step 7: Pour 7kg milk from 8kg barrel into 3kg to 3kg barrel, that is, 7-7kg remains in 8kg barrel.