I had rolled my bike with more difficulty than usual across the grassy field. "You're just tired," I told myself to dismiss the added effort. When I started to pedal homeward an hour later, I realized that the rear tire was -- and had been -- completely flat. Thankfully, a public bus provided adequate back-up transportation.
Having secured requisite repair items a few days later, I set about changing the flat. I located the culprit: a large screw buried up to its head, which I extricated from both tire and tube. I scrunched a new tube home, seated the tire, and pumped in air. I reveled in being back in pedaling business until the next morning when I tested the tire with a squeeze that revealed disappointing softness. I wallowed in a few moments of frustration, and then I repeated the changeover process with a fresh tube. This time, I tried to be more thorough by feeling around the inside of the tube for further vexations. I found one I had not detected previously, when the screw had seemed like the low-hanging (and only) fruit. My finger now felt something poking out, thorn-like, from the inside of the tire. With pliers, I tweezed out what appeared to be a tiny metal hair and finished the fix. The tire seemed reassuringly firm the next morning, so I rode. A quarter mile from my destination, however, the back-end of the bike clanked, followed by the uncomfortable grind of wheel rim on pavement. Another flat.
For the third and proverbially charmed time, I took the bike to a professional, explaining my saga so far. The mechanic set me up with a thicker thorn-proof tube and proclaimed both rim and tire free of any threats. So far, so good -- if only because I now carry a spare tube, pump, and tire irons with me on each commute.
Some morals of the story: Pay attention because the obvious problem may not be the only problem, and, should problems persist, consider inviting in outside eyes. Bringing in a professional, though, doesn't absolve us of responsibility to control what we can control.