"A few years ago I was sitting on a rooftop in New Delhi giving a little presentation. One of the chaps there was a doctor who nodded his head to say "this is just too obvious" while his wife, sitting at his side, just did not get it. I explained it twice then the doctor turned to his wife and said "Villi is like seaweed in the ocean. They wave in the water drawing nutrition into the plant. A storm then comes through, churns up the water, rips up the seaweed and throws it up on the beach. When the storm is over things settle down the seaweed re-grows and things go back to normal."

Or maybe they are like the roots of plant. They are a structure that has a very large surface area so that there is a maximum amount of chemical uptake.