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: gills are used for breathing water by aquatic animals. 
Sentence2: gills are used for breathing water by sharks.

SOLUTION: are

PROBLEM: Sentence1: An accounting period is simply a period of time. 
Sentence2: seconds are used to measure a period.

SOLUTION: period

PROBLEM: Sentence1: Sperm had to swim from male to female reproductive organs for fertilization. 
Sentence2: Sperm and eggs are cells known as gametes.

SOLUTION:
Sperm