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: Plants exhibit phototropism, or growing toward a light source.
Sentence2: Phototropism means growing toward the energy of light.
Example Output: phototropism

Example Input: Sentence1: the mass of a planet causes the pull of gravity on that planet.
Sentence2: the mass of a planet is what causes things to have weight.
Example Output: mass

Example Input: Sentence1: Antigens are found on cancer cells and the cells of transplanted organs.
Sentence2: Organs such as the stomach and liver also begin to develop.
Example Output:
organs