system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens if you break the sound barrier underwater?
gpt
The water vaporises at the tip of the thing going fast. As the steam bubble expands, the fast thing starts moving through the big bubble created by the expanding vapour. This causes the fast thing to become surrounded by the bubble of steam created at the tip since it is moving through the expanding bubble faster than the surrounding water can cool the steam and return it to a liquid state. That allows the fast thing to move through steam, rather than water.