You and a friend go to a garden shop to buy fertilizer for your new plants. Your friend says, “Didn’t we just learn that plants make their own organic molecules through photosynthesis? Plants shouldn’t need fertilizer.” Another customer overhears your discussion and looks at the bag of fertilizer you are holding, “Well,” she says, “it looks like the main ingredients in fertilizer are nitrogen and phosphorus. I don’t know if plants really need that stuff or not.” What do you say? Do you buy the fertilizer?