Well, this Mind Bender stumped all but one person — Daniel Hayden. So, well done!
Remember … you had to balance 12 nails on top of a single nail, hammered into a block of wood.
You start by laying one nail on the table, and on it at 90 degrees (ie: perpendicular) lay all the other nails (except for one of them) alternately each side, with their points furthermost.
There must be five nails on each side, and they must fill the length of the nail on which they lay, so adjust the quantity of nails to ensure this happens. (Now you see why you need to practice this.)
Lay the last nail on top of the first nail pointing the opposite direction (the last nail is shown in red on the second diagram).
Then … gently lift the assembled nails by holding the ends of the top and bottom nails. As the perpendicular nails sag down to an angle of around 45 degrees, amazingly the whole assembly locks itself together.
You can now balance the assembled nails on the one supporting nail in the block. (The balance point is extremely forgiving, due to the counter-balancing effect of the nails hanging down lower than the point of support.)
The final diagram shows you a side view of how it will all come together.
And now you can see a photo of how the finished product should appear in real life.