You have a 12 litre jug, an 8 litre jug, and a 5 litre jug. None of the jugs have any markings on them. The 12 litre jug is full, and the other two are empty.
How can you divide the 12 litres of water in such a way that two of the jugs (the 12 litre and the 8 litre jugs) have exactly 6 litres of water in them, and the third (the 5 litre jug) is empty?
Fill the 8 liter jug with the 12 liter jug, leaving 4 liters in the 12 liter jug.
Fill the 5 liter jug with the 8 liter jug, so that there are 3 liters remaining. Empty the 5 liter jug into the 12 liter jug. Now there are 9 liters in the 12 liter jug and 3 liters in the 8 liter jug.
Pour the 3 liters from the 8 liter jug into the 5 liter jug. Now fill the 8 liter jug with water from the 12 liter jug, leaving 1 liter in the 12 liter jug.
Fill the 5 liter jug (which already has 3 liters in it) from the 8 liter jug, leaving 6 liters in the 8 liter jug.
Empty the 5 liter jug into the 12 liter jug. Now there are 6 liters in the 12 liter jug, 6 liters in the 8 liter jug, and the 5 liter jug is empty.
Also, DJ presented a table here that shows the seven steps to the simplest solution: