Here's a question that probably has a fairly (and ridiculously) simple answer:
How do the Weasley twins know how to operate the Marauder's Map?
There obviously aren't instructions on it. I highly doubt that any of the Marauders bothered to leave instructions with Filch on how to use the map. Using a revealing charm (as Snape tried to do) doesn't seem to work (unless that was unique to Snape, considering the enmity between him and the Marauders). And none of the Marauders were around to explain to Fred and George how the map was used.
What really frustrates me is I just finished a read through PoA. So I feel kind of silly for missing it, if the answer is in the book in a fairly obvious setting. But it's bothering me enough to wonder--how did they figure out how to use that map?
(...I just know I'm going to kick myself when I learn the answer...)