sentences: a comet is often made of ice
docs: interstellar_comet is exosolar_comet. comets has solid_nucleus comets has ice comet is extraterrestrial_object. short_period_comet is periodic_comet. long_period_comet is periodic_comet. comets is like_dirty_snowballs. periodic_comet is comet. comet is icy_planetary_body. comet is dirty_iceball.