That's correct. Usually it's a piece of glass embedded in the tire, sometimes small pieces of metal
like the pointy part of a thumb tack. But there were 2 occassions that I couldn't find the debris that
caused the punctures. I hate it when that happens, because if I patch the tube and
if I missed the cause of the puncture and it's still there; it might give me a flat again. I'm thinking that
it maybe possible that whatever got my tire may have been embedded in the asphalt or that it may
have fallen off after a few turns of the wheel.