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: creating wood-free paper does not require cutting down trees. 
Sentence2: creating printed media requires cutting down trees.
creating

Sentence1: propulsion is used for flying by a spacecraft. 
Sentence2: Jets are used for flying by spacecraft.
flying

Sentence1: a firecracker converts chemical energy into sound. 
Sentence2: Hearing is the act of perceiving sound.

sound