What do pebbles, problems, pearls, and perseverance have in common? Okay, the p alliteration, but what else? I thought about this question as I hobbled along with a small rock in my tennis shoe. In case you’re wondering why I didn’t take off my shoe and get the pebble out, the seventy-nine pound lab at the end of the leash I held was not interested in stopping. My discomfort brought oysters and pearls to mind. I’d always believed pearls were formed from the irritation of a grain of sand, but according to research, parasites or other organic materials are the more likely culprits. Interestingly, like the pebble in my shoe…