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: Eggs are usually laid in a nest. 
Sentence2: birds have nests.

SOLUTION: nest

PROBLEM: Sentence1: a flashlight converts chemical energy into light energy. 
Sentence2: Chemical energy converted to light energy requires batteries.

SOLUTION: chemical

PROBLEM: Sentence1: Water freezes at zero degrees Celsius. 
Sentence2: When water reaches zero degrees Celsius, it expands.

SOLUTION:
Celsius