Dinner from the Yard

Agriculture is the art and science of cultivating the soil: I make my own soil. Science and the World provides.
I asked Pist who is a scientist: I can't drink rain water without becoming very ill. How is it I can eat plants that absorb the rain?
Pist: Plants filter the water. Me: Thats' it, that's your answer??? Pist: I'm in a meeting. The short answer is science.