Lee had calculated that by the time the ant had crawled up from his thumb onto the first brass button, the felon who was inside robbing the shop would be ready to leave. He also calculated that by the time the ant had reached the second brass button the felon would be rushing out of the door. He further realised that by sticking out his foot he could trip up and arrest the villian and if the thief should get away, oh well, there's always another day.