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.

Input: Consider Input: Sentence1: Plants exhibit phototropism, or growing toward a light source. 
Sentence2: Phototropism means growing toward the energy of light.

Output: phototropism


Input: Consider Input: Sentence1: melting is when solids are heated above their melting point. 
Sentence2: Ice is a solid, it's cold and it's hard.

Output: solids


Input: Consider Input: Sentence1: contact between rocks over long periods of time causes rocks to smooth. 
Sentence2: Sandstone, shale, and limestone are sedimentary rocks.


Output: rocks