Wednesday, 19 November 2008

Brain teaser

This is the start of a new series of mind puzzles, shamelessly lifted from a book I bought recently. See how you go with this one, which is the first in the book.

Love potion
Merlin has to make a love potion for King Arthur. According to his book of spells, he needs 4 litres of 'oil of toad'. To measure this out, the wizard only has two unmarked pots -- one that holds 5 litres, the other 3 litres. How can he measure out 4 litres?


  1. Can I leave an answer? Or are you planning to give the answer later?

  2. Leave an answer by all means! If nobody gets it, I'll post the answer in a few days.

  3. In that case, my answer is that he fills the 5L pot first and then uses this to fill the 3L pot. He puts the remaining 2 litres from the 5L pot aside and then repeats the process to give 4L. Am I right? ;)

  4. Close . . . except there is no vessel in which to put the 2 litres aside . . .

  5. Ok. I’ll bite – can’t resist a puzzle, elementary as this one is.

    My initial thought was the same as Hayley’s – naturally I assumed the wizard would have a cauldron laying around (it was only after you disallowed Hayley’s answer that I realised my flaw. It’s witches that have cauldrons, not wizards).

    Anyway, fill the 3 litre flask & empty it into the 5 litre flask.
    At this point, the 5 litre flask can take another 2 litres.
    Fill the 3 litre flask again & empty as much of it will fit into the 5 litre flask.
    The 3 litre flask now contains 1 litre.
    Drink all 5 litres of the toad oil in the 5 litre flask – seems a shame to waste it.
    Now, pour the contents of the 3 litre flask (1 litre) into the 5 litre flask.
    Fill the 3 litre flask again & pour it into the 5 litre flask.
    Voila. Four litres in the 5 litre flask & a pile of vomit your feet.
    (Not even wizards can keep down 5 litres of toad oil.)

  6. LOL

    I think that works, although the official answer has Merlin only drinking 3 litres of the toad oil instead of 5.

    ie you fill the 5 litre flask, pour it into the 3 litre flask (leaving 2). Discard (or drink if you prefer) the 3 litres and transfer the 2 litres into the 3 litre flask. Fill the 5 litre flask again and fill the 3 litre flask (takes 1 litre), leaving 4 litres in the 5 litre flask.

    You'd probably still have a pile of vomit though.

  7. don't you love how there's always more than one way to skin a cat.