When I was eight-years-old, I had a convulsion due to high fever.  Before I was released from the hospital, and even after, I was given electroencephalographs to determine if there was any lingering problems/abnormalities.  A unusual and interesting side effect of the electroencephalographs was that I couldn't feel electrical shocks for several years afterward.  

Since we lived in the country, it meant I could grab ahold of my neighbors electric fence and step over it, but when anyone was with me, they would get a shock and unusually jump back and start swearing.  They would also want to know why I didn't warn them about being shocked, and I'd have to tell them I didn't feel a thing.  Of course, most wouldn't believe me and thought I'd set them up on purpose, which sometimes I had, but not feeling it myself it wouldn't dawn on me what would happen to them, and this was all do to the electroencephalographs I'd had.  

4 hours ago, drpaladin said:

How small is a pebble?

Pebbles can range from the size of a marble to the size of a golf ball.  David would have used one about the size of a golf ball when he slew Goliath.  

There are some people you don't need an electroencephalograph to tell that the lights are on, but nobody is home.