In 75% of my flats, i have not been able to find a cause. This makes me very nervous, because i usually carry only one spare tube. I got one this past weekend. I found the puncture in the tube, and I knew where it should be on the tire. I checked and checked and found nothing. I put in the new tube, and it is holding. It just makes me wonder what caused the puncture. It had to be sonething that went through the tire and tube and came back out. I wonder what could do that. I have seen pieces of a steel belt from a car tire. They're thin and sharp.