*By Michael Hartley*

- fill up the 3 gallon tub. Now, the 3 gallon tub is full, and the 4 gallon tub is empty.
- pour the 3 gallon tub into the 4 gallon tub. Now the 4 gallon tub contains 3 gallons.
- fill up the 3 gallon tub. Now both tubs contain 3 gallons.
- top up the 4 gallon tub from the 3 gallon tub. Now the 3 gallon tub contains 2 gallons!

In fact, sometimes it means that the puzzle can't be solved. Try making 7 pints with a 3 pint tub and a 9 pint tub, for example! There's a very simple rule to determine what solutions are possible, for given tub sizes :

**Work out the greatest common factor of the tub sizes. You can make any multiple of this, up to the size of the biggest tub.**

Not sure how that works? The form below will work out for you what amounts of water you can make up, for two given tubs.

Tub 1 : | units | |

Tub 2 : | units | |

Here's a playable version of this game. You can see two cups, and a tap. Move water around by clicking on the tap or the cups - click first on the place you want to take water from, then click on the place you want the water to go to.

Difficulty : |

You don't need to be online to play this game. Draw some cups or buckets on a piece of paper, and use a pencil to keep track of how much water is in each bucket. As long as the goal follows the rule I mentioned earlier, the puzzle has a solution. If you aren't sure, here's some suggestions. I've rated each suggestion according to how difficult it ought to be. If these are not enough, you could always click 'start new game' above and record the goal and the volumes of the cups.

- Make 2 litres, with a 3 litre bucket and a 4 litre bucket (easy)
- Make 3 gallons, with a 5 gallon drum and a 7 gallon drum (easyish)
- Make 2 ounces, with a 5 ounce cup and an 8 ounce cup (moderate)
- Make 5 mL, with a 10 mL spoon and a 13 mL spoon (tougher)
- Make 12 pints, with a 24 pint barrel and a 29 pint barrel (tougher still)

The game can also be played with three containers - in the puzzle below, you'll always need to use all three cups to reach the goal.

Difficulty : |

In reality, this is not a practical way to get the right amount. If you make even a tiny error with each pour, these errors will add up, so that at the end, the volume you have is completely random.

(By the way, the nice photo of a tap was not taken by me. It's called Memories of summer, and was taken by flickr user tetu. Thanks, tetu, for making this pic available for use under a creative commons licence!)