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: Batteries are a better source of DC electricity. 
Sentence2: a flashlight requires batteries to produce light.
Batteries

Sentence1: Spikes are forceful the tidal waves of electric current. 
Sentence2: spikes come from tidal energy.
Spikes

Sentence1: harming an animal species has a negative impact on the population size of that species. 
Sentence2: harming an animal species has a negative impact on the sub-units of species.

animal