This first part of this puzzle is quite well known, but is worth another look, as some people might not have seen it before. The second part is a little twist that makes the puzzle slightly more difficult, and it is probably much less familiar to the majority of people.
First, you have to measure exactly four pints of water in a jug. The complication is that you have only two jugs; one that holds three pints and another that holds five pints. Neither jug has any markings, and both are irregular enough in shape that you can't estimate what proportion of the jug is full. That is, you can't work out what four-fifths of the five-pint jug looks like.
You can fill either jug from a tap as many times as you want.
So, how do you get four pints of water in the larger jug?
And now to the twist. In this part of the puzzle the five-pint jug is too large to go under the tap, so you can only put water in the five-pint jug by pouring it from the three-pint jug.
Again, how do you get four pints of water in the big jug?
Answers at 9.00 on Monday