Megillath Esther (4:3) states that
in every province, whithersoever the king's commandment and his decree came, there was great mourning among the Jews, and fasting, and weeping, and wailing; and many lay in sackcloth and ashes.
On its face, I would be inclined to imagine that, whenever the messengers arrived at a new location with the official proclamation of the new law, the Jews there suddenly wept and mourned.
Apparently this is exactly how Rashi understood it. Baruch SheKivanti.
Only, I understood it this way not just as Pshat (the simple meaning), but as the obvious explanation, without Rashi's prompting, in an age of internet, smart phones, and social media. Rashi, on the other hand, lived in a time not so technologically different from ancient Persia. So why did he think the reader of the Megillah might not get that this is how word traveled back then?