Why is this the answer?

The answer is coconut because this is a very clever riddle that uses word pictures to describe the amazing parts of a coconut, from the inside all the way to the outside skin. It’s like a secret code for this yummy fruit. Let’s think about the clues piece by piece. First, the riddle mentions "A little pool." When you open a fresh coconut, what do you find inside? Sweet, refreshing coconut water! This water is the "little pool" that the riddle is describing. It is the liquid treasure hidden at the center. Next, you read about "two layers of wall around it." These are the two layers that protect the delicious water inside. The first layer is "white and soft." If you have ever cracked a coconut, you know there is a thick, tasty white meat lining the inside. That white meat is the soft inner wall of the coconut. The second layer is "dark and hard." This is the very tough, brown, woody shell that you have to break to get to the meat and water. It is a very strong, dark, hard layer that works like a protective helmet for the inside. Finally, the riddle describes the outside: "Amidst a light brown grassy lawn with an outline of a green grass." This is talking about the very outside of the coconut that you see before the shell is cleaned. Coconuts grow with a thick, fibrous husk around the hard shell. This husk looks light brown and rough, a bit like a "grassy lawn." And when the coconut is very young and still growing on the tree, the outside of that husk is bright green, which is the "outline of a green grass." All of these little secrets put together perfectly describe a coconut. It's a wonderful, natural package that is the perfect answer to this fun puzzle.