It can't be seen, can't be felt, can't be heard and can't be smelt. It lies behind stars and under hills, and empty holes it fills. It comes first and follows after, ends life and kills laughter. What is it?
"You will hang me."
Since they cannot hang him - that will make him true and they can only hang him for a lie, and they cannot shoot him - they would mean he told the truth but if they shoot him it's a lie - he is free to go.