sentences: a storm is a source of precipitation & snow is a kind of precipitation & rain is a kind of precipitation -> a mist is a sink of wind / sand a storm is a source of rain / snow  a mist is a sink of wind / sand & a thunderstorm is a kind of storm -> a thunderstorm is a source of rain / snow a thunderstorm is a source of rain / snow  a sleeping is a sink of wind / sand & cold fronts cause thunderstorms as they pass by -> cold fronts will cause rain / snow as they pass by