a flower produces pollen and seeds & if something produces something else then that something is a source of that something else ->  a flower is a source of pollen  bees eat pollen & a bee is a kind of living thing & living things eat their food source ->  pollen is a food source for bees  a flower is a source of pollen & pollen is a food source for bees ->  a flower is a source of food for bees by producing pollen  if a thing converts something into something else then that thing uses that something & bees convert nectar into honey for food ->  bees use nectar for food  bees use nectar for food & a flower is a source of nectar ->  a flower is a source of food for bees by producing nectar  a flower is a source of food for bees by producing pollen & a flower is a source of food for bees by producing nectar ->  a flower is a source of food for bees  a flower is a source of food for bees & one can get something from a source -> bees get food from flowers