Bay seniors interested in dumbfounding their friends and family might consider taking up the precarious art of rock balancing. These slanting, gravity-defying, Pixaresque sculptures look like they are about to crash down any second. They are very carefully created in what their makers, a global community, call “working meditation”. There’s a class this Saturday from 11 to 1 at Crissy Field just east of the Warming Hut. It’s free but reservations are requested. And yes, sooner or later, your sculpture will fall down.