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: when a gas in an open container evaporates , that gas spreads out into the air. 
Sentence2: If you place gas in an open box it will evaporate and spread out into the air.

A: gas
****
Q: Sentence1: protein is used to repair cells by the human body. 
Sentence2: Wound healing requires protein.

A: protein
****
Q: Sentence1: a valley is formed by a river flowing. 
Sentence2: Water erodes rocks as they flow over, creating a valley.

A:
valley
****