When you're hard at work, deep in thought or emotionally stimulated, your brain sends signals to your skin, especially your hands and feet. This is called electrodermal activity, and it's triggered by the sympathetic nervous system as part of our innate "fight or flight" response system. Measuring this activity used to require wires and sensors, which had its shortcomings—namely, wearers couldn't wash their hands. Advances in sensor technology now allow for noninvasive monitoring all day and night.