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: plant reproduction requires pollination. 
Sentence2: Plant reproduction requires insects and hummingbirds.
plant

Sentence1: a flashlight requires a source of electricity to produce light. 
Sentence2: Flashlights provide the safest emergency lighting source.
flashlight

Sentence1: All oaks are plants. 
Sentence2: when an oak grows , the size of the leaves of that oak may increase.

oaks