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: Mammalian teeth are important for digestion. 
Sentence2: bats' teeth are important for digestion.

SOLUTION: digestion

PROBLEM: Sentence1: Hydrogen bonds cause water to expand when it freezes. 
Sentence2: Water freezes into ice when cooled.

SOLUTION: freezes

PROBLEM: Sentence1: Invertebrates are animals who lack a backbone or spinal column. 
Sentence2: vertebrates have a spinal column.

SOLUTION:
column