Use Darwin's theory of natural selection to explain how the elephant's trunk evolved

An elephant's trunk evolved in order to be able to stuff in and eat as much food as it can to become the big animal it is today.

The elephant's ancestors were small and had a short nose. As it turns out, the larger body size the elephant has, the more chance it gets to survive and pass on its genes to the next generation. In order to gain a larger size, it needed to eat more and be able to reach more food. Of course, its small nose wasn't capable of doing that at the time. So, through natural selection, eventually the elephants that were bigger, had longer trunks, and therefore ate more, were more likely to pass on their genes to the next generation than their smaller counterparts, and therefore with each generation elephants got longer trunks.