system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't we use stainless steel flask to store whiskey for longer than few days?

It contains a lot of alcohol like 40%+ so any bacteria shouldn't grow inside. Can it poison you or kill you or does it only affect taste or something?
gpt
Alcohol is fermented in stainless steel barrels, so this is a good example of irrational herd behavior.

Most humans act by copying other humans that have been successful at something.  When humans copy something they rarely understand *why* they're doing it, they just understand that it "needs" to be done.  What this means is that sometimes they copy things that don't actually have any meaning and don't actually have to be done.  A good example of this is the "this movie does not depict any real people" disclaimer that you see at the end of the movies.

That disclaimer does literally nothing.  It came into existence as a result of a poorly worded legal opinion out of a California court 100 years ago.  Essentially, some studio executives who didn't understand the opinion started putting that disclaimer in the credits to their movies and everyone has just copied it since.  

This copying of the disclaimer is so prevalent that its extremely common to see it pop up in foreign media that will never be shown in the US.  For example - just about every German TV show contains this disclaimer in their credits even though there's never been anything in Germany related to it.  The reason its there is because German TV producers learned how to make TV shows by copying American TV shows and since that disclaimer was in all of the American TV shows it ended up getting copied as well.

So why do those flasks say not to store alcohol in them?  Because at some point some manufacturer put that warning on a flask.  Other manufacturers then saw the warning and copied it because they assumed that it was there for some purpose.  And now you have a bunch of people trying to back rationalize why this warning is on every alcohol flask.  This is a relatively recent thing and is mostly likely the result of a large Chinese manufacturer who saw warning labels on other US products and didn't understand why they were there.

But there is no reason for the warning.  If alcohol was going to cause some reaction with the flask then it wouldn't matter how long you stored it in there - the flask wouldn't be safe for you to drink out of at all.  

And again, the fact is that alcohol is fermented in stainless steel barrels.  There is nothing special about those barrels.  If those barrels are safe to store alcohol in then so is your flask.