TASK DEFINITION: Generate an overlapping word between the given two sentences. When you find the overlapping words, they don't have to match exactly, e.g., "survival" and "survive" are valid overlapping words. Little words like "the" or "of" don't count! You must generate significant words which are not the stop words.
PROBLEM: Sentence1: an image in a mirror is formed by reflecting light. 
Sentence2: mirrors can be seen with our eyes.

SOLUTION: mirror

PROBLEM: Sentence1: waxy leaves are used for storing water by some plants. 
Sentence2: waxy leaves are used for water conservation by some plants.

SOLUTION: leaves

PROBLEM: Sentence1: Mutations are essential for evolution to occur. 
Sentence2: Natural selection is the mechanism of evolution.

SOLUTION:
evolution