Here is a pretty good article by my friend Frank Turek at The Stream
I grew up on the Jersey shore. (No, it wasn’t like the TV show.) Every summer morning I’d see several men combing the beach with metal detectors looking for jewelry and change lost the day before. One lost diamond earring or ring could pay for the metal detector several times over.
But as useful and successful as metal detectors are, they can’t be used to find everything. Metal detectors won’t help you find wood, plastic, rubber, or other nonmetallic objects.
Now, suppose metal-detector man, after just combing the beach, says to you, “I know there’s no plastic or rubber on that beach because I looked for those things with my metal detector and found nothing!” Then suppose he goes even further and says, “There’s not only no plastic or rubber on that beach, there is no plastic or rubber anywhere because I’ve never found a speck of it with my metal detector!” Meanwhile, you can’t help but notice that his metal detector is made of mostly plastic and rubber.