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.

Sentence1: Graphite and diamond are polymorphs of carbon. 
Sentence2: carbon polymorphs are used for making pencil lead.
carbon

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

Sentence1: sexual reproduction increases genetic diversity. 
Sentence2: The conclusion is that genetic diversity is desirable for the health and long-term survival of a population.

diversity