Three men are sentenced to death by the firing squad.

The first one is tied to the pole, the firing squad marches into position.
At the moment they take aim, the sentenced man calls out "Earthquake!", and the firing squad flees in panic.
The man is released, as no one ever before had the idea of using that idea.

The second one is tied to the pole, the firing squad marches into position.
At the moment they take aim, the sentenced man calls out "volcano outbreak!", again the firing squad flees in panic.
The second man is released as well, same reason - no one before had the volcano idea.

So the third one gets tied to the pole, again the firing squad marches into position.
The third man thinks hard what catastrophy he could use to get free as well, but can't think of anything.
So the firing squad takes aim, and in that moment, the third man at last has an idea, and he calls ...

"Fire!"