Put your bare foot down on the ground and you'll connect with the earth's electricity. Any type of ground works: grass, sand, gravel or concrete. Or swim in a lake (my personal favorite; see below). Or lean against a tree.
It's called grounding. Idea being that we humans have lost our connection to the earth's electric charges since we wear shoes (with good reason) and thus our ability to access the negative charge generated by the earth. These negative ions can act as antioxidants which neutralize free radicals. Research shows that barefoot contact with the earth can actually produce changes in physiological measure, help improve sleep, reduce pain, decrease muscle tension and lower stress.