[MENTION=496584]ClydeClydeson[/MENTION] makes an excellent point. Not all flats are from some sharp object on the road coming through the tread. With a Classic or Vintage bike sometimes it is the rim, spoke or spoke hole that causes the problem over time. The other non-intrusion flat to be aware of is called a pinch flat. I call them "snake bites". This is where hitting a pothole, root or some other object cases the tire to get pinched to the point that it squeezes the sidewall and leaves two punctures in the side of the tube. Proper inflation and avoiding objects (if you possibly can) will minimize these type of flats.