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.

[Q]: Sentence1: Osteoporosis is a disease characterized by a decrease in bone mineral density. 
Sentence2: a disease in which osteoclasts are more active than osteoblasts results in a decrease in bone mineral density.
[A]: bone


[Q]: Sentence1: a light bulb is used for seeing in the dark. 
Sentence2: light is required for seeing.
[A]: seeing


[Q]: Sentence1: water causes refraction of light. 
Sentence2: water can change the speed of light.
[A]:
water