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.

Example Input: Sentence1: triceps contracting causes a person 's elbow to straighten.
Sentence2: muscles contracting cause a person's elbow to straighten.
Example Output: contracting

Example Input: Sentence1: Blood pH may become unbalanced if the rate of breathing is too fast or too slow.
Sentence2: If Someone Is Hyperventilating Hyperventilation is caused by breathing too deeply, too fast.
Example Output: breathing

Example Input: Sentence1: Each trophic level has less biomass and a loss of energy content.
Sentence2: energy decreases as it goes up levels.
Example Output:
energy