I'm busy

On Mondays I stick around the office during lunch time. I run a few errands online, do some light reading, blog. It's my own time. Sometimes co-workers come and chat and that's fine, as long as it's not about work. But what picks my ass is when I'm glued to some webpage, obviously reading something that I find interesting and someone starts to have a full on conversation with me! Do I ignore them? Of course not, I smile, nod and provide a short acknowledged response and quickly continue my reading. Most don't seem to read the body language well. I don't see why it's hard to tell that when I barely looked at you to smile and I'm still looking at the screen, I don't want to be talked to. With CB I'm much more rude. I simply ignore his comment and respond by leaning towards my screen and begin reading out loud. On a pissy day I'll begin to read out loud before he completes his sentence. That's pretty obvious right? Now how do I translate that to other people? I should bring earphones and pretend I can't hear them.