It's a sweltering summer afternoon. I'm pushing aside tree limbs and crunching leaves to get back to the trap that I baited two hours ago with dry ice to attract ticks. When I get closer, I can see a gossamer mist hovering over a bright white cloth in the dark underbrush. Dry ice "sublimates" in the open air, going from a solid to a gaseous state. It gives off a vapor of carbon dioxide gas that's denser than the air, mimicking the breath of a tick host resting on the ground.