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.

Q: Sentence1: Mollusks are invertebrates such as the common snail. 
Sentence2: snails have no backbone.

A: snail
****
Q: Sentence1: Fish are the first known true chordates. 
Sentence2: Fish are the first known animal with a backbone.

A: Fish
****
Q: Sentence1: Many birds perform elaborate courtship dances. 
Sentence2: Many birds perform dances before they mate.

A:
Many
****