Being witty about poisoned drinking water isn't easy. It requires a certain obtuse savoir-faire. Our president gave it a go Thursday night at a press dinner here. "As you know, we're studying safe levels for arsenic in drinking water," he told the crowd of radio and TV correspondents at the Washington Hilton. "To base our decision on sound science, the scientists told us we needed to test the water glasses of about 3,000 people. Thank you for participating." What an idiot.