The researchers taped absorbent pads to the armpits of 40 volunteers about to do their first ever sky-dive. They collected the sweat produced as the volunteers plummeted to earth and then asked a second ground of volunteers to breathe the fear-soaked samples alongside some fear-free sweat. The second group’s brain activity was monitored, using a scanner, as they smelled the samples and they displayed more activity in the brain’s fear centers when they were exposed to the skydivers sweat.