Just to prove they could, the engineers created tiny flying bots. Gnat small. Mote of dust small.
The bots were programmed to buzz around until they came across another tiny bot. Once two bots hooked up, they buzzed around until they found some sand. Just a few grains would do. After using the sand to replicate, all three bots buzzed away and started over.
Then someone became careless: an open window, a hearty sneeze, and before you could say Isaac Asimov, the bots escaped. Soon the air was so full of tiny bots that no one could breathe.
The end, literally.