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: a solution is formed by one substance being dissolved in another substance. 
Sentence2: A solution is formed when limestone is dissolved in ocean water.
formed

Sentence1: creating paper requires cutting down trees. 
Sentence2: Trees begin as epiphytes on host trees.
trees

Sentence1: the volume of an object can be used to describe the size of that object. 
Sentence2: the volume of a crystal ball can be used to describe the size of the crystal ball.

be