Also I always wondered how the twins knew how to work the map. Even Snape couldn’t work it out, how did they know what to say to get it to work?
The password to the map was ‘I solemnly swear I am up to no good’. Knowing the twins, there was a good chance they would say that even without reference to the map. No, this is not a serious answer.
The most likely answer is that they used trial and error, or that the map opened up to them, seeing in them fellow mischief makers, and perhaps told them or helped them.
And it’s always possible it was some sort of school legend, passed down from menace to trickster through the generations (the password, that is). Filch certainly didn’t confiscate it from the Marauders themselves (seeing as he wasn’t around at the time they were), and therefore there had to be some older pupils or alumni with the secret.
Duke