Me whats the secret? we have a secret, just we three, the robin, and i, and the sweet cherry-tree; the bird told the tree, and the tree told me, and nobody knows it but just us three. but of course the robin knows it best, because she built the—i shan't tell the rest; and laid the four little—something in it— i'm afraid i shall tell it every minute. but if the tree and the robin don't peep, i'll try my best the secret to keep; though i know when the little birds fly about then the whole secret will be out.

The secret is that the robin made the tree. What has me believe so, is because the robin said she knows the secret best. "Because she built the—I shan't tell the rest; And laid the four little—something in it—" and Then say,"Though I know when the little birds fly about Then the whole secret will be out."  You can fly out of a tree! The secret is also between the tree and the bird. This references the tree often which makes me truly believe that this is the answer.

onto a large boat, and mark the water level that the boat sinks to. then, take the elephant out and put a bunch of marked weights onto the boat until it reaches the same water level. add up the masses of the weights to get the weight of the elephant.