system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does dunking a hot metal in cold liquid make it harder?
gpt
Iron and steel can have a variety of different internal structures - the exact nature of the structure you get depends on a few things, but in iron and steel two of the things that have the biggest influence are carbon content and temperature.

Carbon content is a relatively easy one to alter, as it doesn't change significantly as the metal cools, but what can you do if you've got a desirable internal structure that's present only at high temperatures?

You can quench the metal... that's what. This rapid cooling means that the transition from the desirable structure to less desirable (i.e. softer) structures doesn't happen to the same extent, and so you end up locking in a structure that you'd normally only see at a much higher temperature in a metal that's now at room temperature.