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.
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Question: Sentence1: Gravity is a force that pulls on things. 
Sentence2: A force that pulls on things pulls objects towards planets.

Answer: force


Question: Sentence1: Ice is frozen water. 
Sentence2: ice expands.

Answer: Ice


Question: Sentence1: Cells are the microscopic building blocks of organisms. 
Sentence2: Cells are the microscopic building bricks of organisms.

Answer:
Cells