Simplest solution yet.
Lay this on a plank of wood. John can pick it up the plank and marvel as Bob collects his beer.
If you want to get technical about straight lines, he could flip the nail the other way, and use a plank to jam the 'head' of the nail into the hammer shaft to form a straight angle, or simply use the hammer to bend the nail head to the desired angle before producing this solution.
