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.

[EX Q]: Sentence1: Acids dissolve in base and turn blue litmus red. 
Sentence2: acid changes color.
[EX A]: Acids

[EX Q]: Sentence1: harming an animal species has a negative impact on the population size of that species. 
Sentence2: Pesticides have a negative impact on the population size of a species.
[EX A]: impact

[EX Q]: Sentence1: a calculator is used for adding. 
Sentence2: Around 500 BC, the abacus, the first mechanical calculator, was developed.
[EX A]:
calculator