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.

Q: Sentence1: Mammals generate heat mainly by keeping their metabolic rate high. 
Sentence2: food keeps mammals' metabolic rates high.

A: Mammals
****
Q: Sentence1: Smoking accelerates the aging process. 
Sentence2: smoking accelerates the death of cells.

A: Smoking
****
Q: Sentence1: Rivers are usually larger than streams. 
Sentence2: streams are smaller bodies of water.

A:
streams
****