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: triceps contracting causes a person 's elbow to straighten. 
Sentence2: muscles contracting cause a person's elbow to straighten.

Answer: contracting


Question: Sentence1: ultraviolet light causes sunburn. 
Sentence2: Sunburn risk is higher at elevation.

Answer: sunburn


Question: Sentence1: when a hemisphere is tilted away from the sun , that hemisphere receives less direct sunlight. 
Sentence2: Sunlight is delivered in small packets of energy called photons.

Answer:
sunlight