Fill the 5-liter kettle first, then pour it into the 6-liter kettle, then fill it with 5 liters, and then fill it with 6 liters, so that the 5-liter kettle has 4 liters of water left. Then pour out the water from the 6-liter kettle, pour 4 liters of water from the 5-liter kettle into the 6-liter kettle, then fill the 5-liter kettle and pour it into the 6-liter kettle. In this way, because the 6-liter kettle used to have four liters of water, now it can only pour two liters. There are only 3 liters of water left in that 5-liter kettle.