WASHINGTON — Justice Sandra Day O'Connor holds popcorn breaks in the afternoon. She takes her clerks and their young families on field trips - picnics at the Tidal Basin while the cherry trees are blooming, hikes in the Shenandoahs, or lunch with the Federal Bureau of Investigation director."She's very demanding, but in the nicest possible way," says Peter Huber, former clerk to Justice O'Connor. Saturdays are for hashing over the upcoming oral arguments. She likes to draw out debate among her clerks, to sharpen questions, they say. She often makes everybody Mexican food for lunch. Mr. Huber recalls early in his year as clerk, still nervous, the justice abruptly buzzed him down to her chambers. He walked in, tremulous, the other clerks all assembled, and she presented him an enormous chocolate chip cookie for his birthday.