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: propulsion is used for flying by a spacecraft. 
Sentence2: Jets are used for flying by spacecraft.
flying

Sentence1: Mitosis is cell division. 
Sentence2: mitosis often causes growth.
Mitosis

Sentence1: Cells are the microscopic building blocks of organisms. 
Sentence2: Cells are the microscopic building blocks of living beings.

Cells