“I became obsessed with symmetry. If I stepped on a crack with my right foot, I felt an irresistible urge to step on a crack with my left foot. I added more details until this ritual became all-consuming. If I stepped on a pebble with my right heel, I had to step on a pebble with my left heel. If I missed and stepped on the pebble with the middle of my left foot, I had to step on a pebble with the middle of my right foot while still ‘owing’ my left heel a pebble.” (>>)