Write to us at startingpoint@globe.com. To subscribe, sign up here. Last Friday, my week in hacky sack mania ended just as abruptly as it began, in the office of the orthopedic surgeon who had replaced my left hip in January, staring down at my feet as I confessed that I may have done something kinda dumb. But let’s start at the beginning, the previous Saturday, when I overheard my 16-year-old son telling my wife that all the kids at his school were obsessed with hacky sack.