You are given a small block of wood with a nail hammered partially into it, and 6 more nails of the same size.

real-life/balance.nails New

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Problem

You are given a small block of wood with a nail hammered partially into
it, and 6 more nails of the same size.  How can you balance all 6 of the
extra nails on the head of the first nail?  No tricks, i.e. glue,
magnets or any external substance, can be used.  It is solved
supposedly by the interlocking of these nails.

-- Dave Evans <devans@kxrm.com>

Show solution

Only 6?  I've done it easily with 12 nails.

Start by laying the first nail on a table.  Lay a second nail, at a
right angle to the first, on top so that the head hooks over the shaft
of the first nail.  Do the same with the third nail, except point it in
the opposite direction.  Continue to lay nails, pointing in alternating
directions, in a row on top of the first nail until one nail remains.
Lay that last nail on top, exactly above and parallel to the first nail.
It helps, but is not absolutely necessary, to point the last nail in the
opposite direction of the first nail.

Now pick the whole thing up by the first and last nails.  All of the
other nails will slide so that their points hang downward, but they will
be held at their heads between the shafts of the first and last nails.
The weight of the nails will tend to lock the assembly together.

Because most of the mass of the nails is hanging below the support
point, you can easily balance this assembly on the head of the fixed
nail.

-- Mike Naylor  (December 1997)