Please briefly talk about what is "mosaic" and "theme" according to the poems mentioned in the first and second paragraphs.

Reading interest. Draw out the object of explanation, and vividly explain the formation process and harm of the dammed lake. Stimulate readers' interest in reading. 2. In paragraphs ② and ③, the dammed lake is cited as an example. Please tell us the difference between these two examples. The paragraph gives an example of a dammed lake. Please tell me the difference between these two examples.