system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is some stainless steel magnetic and other stainless steel is not magnetic?
gpt
Iron has two different "shapes" you could say. The atoms within it are aligned in a special pattern, which strangely changes at 911°C from body-centered-cubic (bcc, also called ferrite) to face-centered-cubic (fcc, also called austenite). Interestingly, fcc is not magnetic.

Now for stainless steel. Steel is iron with less than 2% carbon, but that's not necessary to know. To make steel stainless, you need to add at least 12% chrome (or chrome equivalents) that will react with the oxygen before the iron can. This steel has a bcc pattern and is magnetic, because the chrome has a bcc pattern, too.

If you add more than 10% nickel(or nickel equivalents), which is fcc, to the 12% chrome, the pattern in the iron changes from bcc to fcc, even at room temperature. The stainless steel isn't magnetic anymore.

You can look up the "Schaeffler diagram" for some more details(martensite is what you get when you harden steel)

Why would you add that much nickel, when the steel is already stainless? The bcc pattern becomes "brittle" at lower temperatures, meaning it won't stretch before breaking, which is important for safety. Depending on the kind, this can already happen at 0°C. The fcc pattern though doesn't have this effect, it doesn't even care about -200°C, and you can build things like containers for liquid gas out of it.