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: igneous rocks are formed from magma cooling. 
Sentence2: magma solidifies into igneous rocks.

SOLUTION: magma

PROBLEM: Sentence1: Ice is frozen water. 
Sentence2: ice expands.

SOLUTION: Ice

PROBLEM: Sentence1: Marine species live mainly in shallow coastal waters. 
Sentence2: coastal water is calmer.

SOLUTION:
coastal