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: Fish are the first known true chordates. 
Sentence2: Fish are the first known animal with a backbone.

SOLUTION: Fish

PROBLEM: Sentence1: Adaptation traits contribute to individual fitness and to the evolution of animal genetic resources. 
Sentence2: Behaviors can improve the evolution of animal genetic resources.

SOLUTION: animal

PROBLEM: Sentence1: exercise increases a body 's strength. 
Sentence2: Exercise promotes the formation of more mitochondria.

SOLUTION:
exercise