A: Do you like animals?
B: Yes, I always look the TV show about animals.
A: What is the largest animal living on the land? and the smallest animal? Do you know?
B: En, I know the biggest animal is elephant, isn't it? The smallest animals maybe some single cell organism. 
A: You are right. Plankton is the smallest animal in the world. It is a single cell organism. Then what animal is the tallest?
B: I know giraffe have the longest neck, so maybe they should be the tallest one on the earth.
A: yes, they want to eat the leaves on the higher place of tree. So they have to put their neck longer and longer. This was what was known as "the survival of the fittest".? 
B: Yes, the natural is sometimes cruel.
A: You are absolutely right. So, our human beings have to treat the natural well and protect the environment.
B: En, we must care of the preservation and careful management of the environment and of natural resources.?