How to perfecly cut glass bottle #shorts

This guy never skipped physics class to cut a glass bottle perfectly in half. First, he scores a thin line around the bottle with a glass cutter. Then, he alternates dipping it in hot water for a few seconds and cold water right after, and the bottle splits cleanly in two. Glass expands when heated and contracts when cooled. But since it can't flex under sudden temperature changes, it cracks perfectly along the scored