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: Lancelet larvae are free-swimming. 
Sentence2: some larvae lack a hardened backbone.

Answer: larvae


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

Answer: organisms


Question: Sentence1: Aging is associated with the death of cells. 
Sentence2: Apoptosis is a natural part of aging.

Answer:
Aging