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: Mollusks are invertebrates such as the common snail. 
Sentence2: snails have no backbone.

SOLUTION: snail

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: the movement of an air mass over Earth 's surface causes local weather changes. 
Sentence2: Fronts define the boundaries between air masses.

SOLUTION:
air