Before a Buddhist tantric ritual, an assisting monk may swing a censer or thurible as he passes to 'purify' the room. This is a container usually made of metal that hangs form three chains. Inside it, powdered incense that has been put on a smoldering bit of charcoal burns slowly, and the smoke escapes through pierced openings in the closed lid. One tradition says that during one of the Buddha's sermons a monk heedlessly swatted a mosquito. The Tathagatha is said to have ordered that, in the future, incense ought to be lit in order to keep the flies away, so that people could more easily concentrate on Dharma teachings, but also to prevent the needless taking of lives.