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: Diurnal cycles Diurnal cycles are the daily cycles of light and darkness. 
Sentence2: Plants detect and respond to diurnal cycles.
Diurnal

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.
impact

Sentence1: creating wood-free paper does not require cutting down trees. 
Sentence2: creating printed media requires cutting down trees.

creating