It was pouring as I drove my tin-can rental car up the hill outside Tucson.  This is crazy, I thought.  Crazy that it’s raining in the desert, and crazy that I haven’t turned back yet. I listened to the news as I splashed along.  An 85-year-old man, known to have dementia, was missing: went to the supermarket, never returned home. I pulled into the Tucson Mountain lot.  The rain suddenly stopped.  So I grabbed my knapsack and began to follow the first trail I saw. A hundred yards from the car, I hesitated, confused.  The trail had disappeared.  Or rather, there were suddenly half a dozen trails: all formed in the past hour, by rivulets of rain.  Whatever footprints might have once marked the real trail had been washed away.  There was no one else around. This must be what dementia feels like, I thought. I turned around and spotted a stone shelter just above the parking lot.  My beacon: When I turned around, I would head straight for it. I knew I was in no danger, not really, yet I felt queasy: do scorpions come out after a rain in the desert?  Rattlesnakes?  I had no idea.  Could the clouds gather again so quickly and rain hard enough to cause a flash flood?  Probably. I felt small and humble and not very smart.  But I pressed on, thirsty for a little fresh air and exercise.  Twenty minutes, then I’d turn around. 40 minutes later, I made it back to where I started, dry and unbitten [...]