Well, Gus got up to get something to drink, and Floyd sat down on the computer and added a coda to the e-mail. He wrote, and I quote: "I am a butt face". It was the perfect prank--absolutely embarrassing, and maybe even worse to have to explain that your cute little brother pranked you.
The only hitch in this otherwise brilliant plan--Floyd, having never used e-mail, did not know how to send the message, so he was hitting various buttons on the screen when Gus came back in. Gus read the message, screamed, and ripped the keyboard out of Floyd's hands. Gus was rather panic stricken at what almost happened.
I can only surmise that Floyd gets his sense of humor from his mother.