A Tale of Two Lights

...or "How I can be quite clever and stupid in the same five minutes." 

We have a pair of sconces flanking a mirror in our bathroom. A few days ago, the bulb on the right began flickering intermittently. Long ago, a different bulb in the same fixture did the same thing, so I think, "Hmm. Maybe the problem's not the bulb, maybe it's the fixture."

Before disassembling the sconce to look for a loose wire, I want to test the hypothesis. How to determine if the problem is the bulb or the fixture? Swap the right and left bulbs. A while later, the left light begins to flicker. It's the bulb.

I'm still quite pleased with myself as I take the dead bulb to a cabinet, find a replacement, and take them both back to the bathroom. Now I have a bulb in my left hand, an identical bulb in my right hand, and no idea which is which.

Sigh. Pick one at random and set the other one aside. It takes a while, but eventually: flickerflickerflicker. Exchange the bulbs. Three days later, no flicker. Throw away the first bulb, knowing that the moment I take out the trash that sucker on the wall is gonna flicker.