The folks at Human Energy Concealment Systems (HECS) say they know; it's the Electromagnetic Radiation (EMR) produced by the hunter. According to HECS, "It's a scientific fact that every time our heart beats or our body moves, we give off EMR. Scientific research has also proven that animals and humans can detect EMR, particularly when it becomes more intense, or spikes, as is the case when our heart beats faster at the sight or close proximity of game." In response, HECS has produced a line of camouflaged clothing designed to short-stop EMR waves. The lightweight outfit consists of pants, shirt and headnet that contain a "conductive activated carbon in a grid pattern that acts as an EMR barrier." The suit can be worn alone or as undergarment with your normal hunting clothes. Wear it, HECS says, and game will be calmer in your presence.