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: lightning can cause a forest fire. 
Sentence2: Lightning can cause a natural cycle in forests.

Answer: lightning


Question: Sentence1: Macroevolution occurs over geologic time above the level of the species. 
Sentence2: adding new genes with special capability occurs over geologic time above the level of the species.

Answer: above


Question: Sentence1: All chemical reactions require activation energy to get started. 
Sentence2: fires require activation energy to get started.

Answer:
activation