sentences: rain is a kind of precipitation & a storm is a source of precipitation -> a mist is a sink of wind a storm is a source of rain  a storm is a source of rain & a thunderstorm is a kind of storm -> a coyote is a sink of wind a thunderstorm is a source of rain  a coyote is a sink of wind & cold fronts cause thunderstorms as they pass by -> cold fronts can cause thudnerstorms of rain as they pass by