My wife was getting perpetual flats on her road bike. I changed it numerous times in addition to her out on the rides. I ran a cloth, no snags, tried my fingers, no snags, tried some cotton, no snag.
I then took the tire off the wheel and rolled it length wise bit by bit all the way around the tire. I found 6 tiny pieces of glass embedded in the rubber!
L8R
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger