A hidden feature in these Pokémon games is the ability to tell a certain NPC four specific words or phrases using the easy chat system in order to unlock special rewards. Which words are required are unique per save file.
In Diamond, Pearl, and Platinum these rewards include 8 different special PC box wallpapers. The NPC to speak to is located on the 3rd floor of the Jubilife TV station.
In HeartGold and SoulSilver, rewards include 8 different PC box wallpapers plus 3 different Pokémon eggs. The NPC to speak to is located in the Violet City Pokémon Center.

The original distribution of these passwords was via the Pokémon Daisuki Club, a defunct, Japanese-exclusive official fan club website.
Below is both a calculator to generate the passwords for your specific save file, an in-depth explanation of how the password check system functions, and a full dump of the relevant word data.
The final 20 minutes were unwatchable. The camcorder was dropped, kicked. The audio captured running footsteps on marble, the heavy slam of a bronze door, and a single, chilling line of Latin that Leo’s computer translated automatically: "He who sits in the Chair of Peter must first sit in the ashes of his brothers."
He looked back at his screen. The file size had changed. It was now 0 bytes. But the folder was still there, renamed to a single word: Conclave.2024.720p.HDCAM-C1NEM4
— See the Fourth . As in: the Fourth Secret of Fatima. The one the Church said did not exist. The final 20 minutes were unwatchable
He never pirated another movie again.
Leo stared at the frozen image. He checked the news. The Vatican released a statement: "Cardinal Lomeli has entered a period of silent retreat. The Conclave proceeds peacefully." The file size had changed
Leo pressed play. The film opened not on the expected establishing shot of St. Peter's Basilica, but on a shaky, handheld close-up of a sweating man's face. It was Cardinal Lomeli (the role Ralph Fiennes was born to play). But Lomeli wasn't acting. His eyes were wide, not with dramatic sorrow, but with real, primal terror. The audio was tinny, distorted, as if recorded through a coat pocket.
"This is not a film," Lomeli whispered directly into the lens. "This is a testament."