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.

Ex Input:
Sentence1: Many corals secrete an exoskeleton that builds up to become a coral reef. 
Sentence2: Good examples are Bubble coral, Elegance coral, Frogspawn and other Euphyllia corals.

Ex Output:
coral


Ex Input:
Sentence1: a circle graph can be used to display percents. 
Sentence2: pie charts are circle graphs.

Ex Output:
circle


Ex Input:
Sentence1: Flagella help prokaryotes move. 
Sentence2: Flagella are hair-like projections from the surface of cells.

Ex Output:

Flagella