it is hard to tell what caused it since I checkd the tire pressure before leaving home. It must have been a sharp. also, i carry a pump and I inflated the tire after I discovered the flat, but that was good for hald a minute then flat again. I will see if I can Replace the tire myself in the ccomfort of home, and if my back doesn't act up too much (since I have had multiple back surgeries, any bending of weird moves would trigger a back episode). Alternatively! I can just take the bike to a nearby LBS and have him fix it. Maybe I will also change to a narrower tire while I am there :-)