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: Fish are the first known true chordates. 
Sentence2: Fish are the first known animal with a backbone.

SOLUTION: Fish

PROBLEM: Sentence1: drought is when amount of precipitation decreases. 
Sentence2: Drought decreases the availability and abundance of the natural food supply.

SOLUTION: drought

PROBLEM: Sentence1: moss causes chemical weathering. 
Sentence2: moss causes chemical changes in the minerals of a rock.

SOLUTION:
causes