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: microbial fuel cells convert chemical energy into electrical energy. 
Sentence2: microbial fuel cells convert chemical energy into kinetic energy.
[A]: cells


[Q]: Sentence1: Mammalian teeth are important for digestion. 
Sentence2: bats' teeth are important for digestion.
[A]: digestion


[Q]: Sentence1: a greenhouse is used to protect plants by keeping them warm. 
Sentence2: Plants are Earth's producers.
[A]:
plants