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 paper requires cutting down trees. 
Sentence2: Trees begin as epiphytes on host trees.
trees

Sentence1: a stopwatch is used to measure time. 
Sentence2: Time can be measured with a clock or watch.
measure

Sentence1: Muscles can only contract. 
Sentence2: Something that can be smooth can only contract.

contract