Simplest solution yet.
Lay this on a plank of wood. John can pick it up the plank and marvel as Bob collects his beer.
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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.