It’s a genius bit of science, dude. The suit lets a thin layer of water in next to your skin. Your body heat warms that water up, and the neoprene (that rubbery material) traps it there, creating a toasty thermal layer. The suit itself doesn’t warm you—you warm the water. That’s why a good, snug fit is crucial; too loose and all that warm water flushes out with every duck dive, replaced by cold ocean. That flush is a real buzzkill, so seal it up tight at the neck, wrists, and ankles.