It was a mistake to get on the Metro train with the kid riding on my shoulders. I should have taken him down and buckled him into the stroller out on the platform, even if it meant missing the train. But I had taken the wrong branch on the decision-making tree, and there I was, standing up in a packed train car at evening rush hour, with one hand on the kid's ankle to hold him in place, and another hand on the overhead handrail, which meant there was no hand remaining to put on the stroller handle as the train jerked into motion and the clumsily half-set foot [...]