TASK DEFINITION: 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.
PROBLEM: Sentence1: Lancelet larvae are free-swimming. 
Sentence2: some larvae lack a hardened backbone.

SOLUTION: larvae

PROBLEM: Sentence1: freezing means changing from a liquid into a solid by reducing heat energy. 
Sentence2: Water freezes into ice when cooled.

SOLUTION: freezing

PROBLEM: Sentence1: the moon revolving around the Earth causes the phases of the moon. 
Sentence2: the moon revolving around Earth causes different shapes to appear.

SOLUTION:
Earth