Keywords are inclusive, so the more keywords you put in, the less lines you'll find.
You can specifically ask for a keyword to not exist by prefixing it with !.
For example, searching for "mouse !cheese milk" would find lines that contain both mouse and milk but not cheese.

[050425 05:16] f5bd2, Peanut: hmmm

[050425 10:42] 4ef55, Krystal: Wow guys, the room isn't even active and still you bicker. sad.

[050501 01:31] f6591, Kain: *le peek*