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: if fossils of a water animal or plant are found in a place then that place used to be covered by water in the past. 
Sentence2: Common fossils include plant fossils like ferns.

SOLUTION: plant

PROBLEM: Sentence1: if an animal lives under ground then that animal will be protected from a wild fire. 
Sentence2: animals that live under ground in California will be protected from some natural disasters.

SOLUTION: animal

PROBLEM: Sentence1: carbon dioxide concentrations in the air have increased over the last decade dramatically. 
Sentence2: greenhouse gas concentrations in the air have increased over the last decade dramatically.

SOLUTION:
air