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.

Sentence1: gills are used for breathing water by aquatic animals. 
Sentence2: gills are used for breathing water by sharks.
are

Sentence1: a switch is used to start the flow of current in an electrical circuit. 
Sentence2: The flow of electrical current powers a light bulb.
current

Sentence1: Viruses are not cells at all, so they are neither prokaryotes nor eukaryotes. 
Sentence2: Influenza orthomyxoviruses are not cells, so they are neither prokaryotes nor eukaryotes.

are