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: Echinoderms use pheromones to communicate with each other. 
Sentence2: Echinoderms are known for their round symmetry.

SOLUTION: Echinoderms

PROBLEM: Sentence1: Behaviors that improve fitness increase through natural selection. 
Sentence2: Darwinism is strictly, selection theory.

SOLUTION:
selection