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?

You start with a full 12 liter jug. you fill the 8 liter jug to the brim with the 12 liter jug. then you fill the 5 liter jug with the water in the 8 liter jug.

you now have...4 liters in the 12 liter jug...3 liters in the 8 liter jug...and 5 liters in the 5 liter jug.

now you place the water in the 5 liter jug in the 12 liter jug. and then you place the 8 liter jug, now filled with 3 liters, next to the empty 5 liter jug. then using the water level in the 8 liter jug as a mark for the 5 liter jug, fill the 5 liter jug to 3 liters and empty the 5 liter jug into the 8 liter jug

you now have...6 liters in the 12 liter jug...6 liters in the 8 liter jug... and 0 liters in the 5 liter jug