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: Leeches are external parasites. 
Sentence2: Leeches live on the surface of their hosts.
[A]: Leeches


[Q]: Sentence1: Condoms can break; Condoms can be used incorrectly. 
Sentence2: Some methods of birth control can break or be used incorrectly.
[A]:
break