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: Mutations are essential for evolution to occur. 
Sentence2: Genes are altered for evolution to occur.

A: evolution
****
Q: Sentence1: friction is used for stopping a vehicle by brakes. 
Sentence2: friction is used for stopping a car by brakes.

A: brakes
****
Q: Sentence1: Gastropods are snails and slugs. 
Sentence2: Mollusks are invertebrates such as gastropods.

A:
Gastropods
****