A state of being. This can happen in both real life and via text message or IM. It occurs when someone says something to someone else (usually as a joke), and the person becomes really serious about the issue. They stay in this state for quite a while. They reply seriously whenever someone is talking to them.
Indicators of serious mode: Lowercase only in sentences, ending all sentences with periods, one word replies, lowercase "LOL's", slow replies, setting their status as "away" or "busy", signing out of IM, replying with swearing, ending questions with periods, low use of emoticons, using an ellipsis and more.
Phil: Hey what's up George I fucked your mom last night LOL
Bill: ...that's not fucking funny. the fuck do you want.
Phil: Why so serious?
*5 minutes later*
*Bill has signed out*
Phil: That dudes in serious mode.