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

Ex Output:
circle


Ex Input:
Sentence1: Sound vibrations travel at different speeds through different materials. 
Sentence2: the type of material through which sound passes changes the sound vibrations.

Ex Output:
Sound


Ex Input:
Sentence1: Ethologists study animal behavior. 
Sentence2: animal behaviorists study animals in their natural environment.

Ex Output:

animal