Find a fairly bright light source around your home, such as a lamp. The closer your eyes are to the lamp, the better you can see this. A couple feet should work well. Hold up each index finder so they are a couple inches apart, and place them only a couple inches away from one of your eyes. Close the other eye. You should now be looking in the direction of the lamp, through the gap in your fingertips, but your eye must be focused on the tips of your index fingers. Now, slowly bring your fingers together until they touch while continually focusing on the gap between your fingers. Just before your fingers touch, you should notice the gap "close off", almost like your fingers have been drawn together even though in reality they are not yet touching. If you have extremely steady hands, you might be able to "close the cap" and view darkness between your fingers even though they are still half a millimeter or so apart. Apparently, this demonstrates that light does indeed behave like a wave. When light rays are forced through the very small gap, the ridges and troughs of the rays interfere with each other and cancel each other out. You see darkness between your fingers even though they are not yet touching!