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: Fish are the first known true chordates. 
Sentence2: Fish are the first known animal with a backbone.

Ex Output:
Fish


Ex Input:
Sentence1: if soil is permeable then water easily flows through that soil. 
Sentence2: The increase in permeability is caused by the formation and expansion of ice in the soil.

Ex Output:
permeable


Ex Input:
Sentence1: Protozoa can be classified on the basis of how they move. 
Sentence2: Cilia wave in a coordinated way to move the protozoa in one direction.

Ex Output:

Protozoa