So there's this little symbol on things such as tubes of toothpaste, which looks like a little open tin with a number such as "24 M" inside or beside it.
I had always wondered what that was for; my guess so far was that it you squeezed it out, you'd get 24 metres out of it... though why they'd put that measure on it, I have no idea. After all, they already supply the quantity by weight...
Now, by the magic of Wikipedia (German version), I know now that it's the "period-after-opening symbol", used on cosmetics with a shelf life of at least 30 months to indicate "the period of time after opening for which the product can be used without any harm to the consumer". (Different from best-before-end dates since those count from the manufacturing date rather than the date of opening.)
So now I know.
I found it clever that the "M" in that symbol stands for "month" in just about every European language, though, including ones that use the Cyrillic or Greek alphabet.