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: Plants exhibit phototropism, or growing toward a light source. 
Sentence2: Bamboos are vigorous, rugged plants.
Plants

Sentence1: a flashlight converts chemical energy into light energy. 
Sentence2: Only as long as there are batteries in the flashlight, the flashlight will function.
flashlight

Sentence1: Shivering occurs when many muscles contract a little bit all at once. 
Sentence2: Heat production is increased by shivering and non-shivering thermogenesis .

Shivering