Glass Cup Vacuum?

My cousin was eating dinner with me tonight, when he picked up his empty glass cup, and started to suck the air out of it so that it would act like a vacuum and the glass would stay on his mouth. When he took the glass off, it had left an imprint of the rim outside his lips, and a discoloration. It looks like a bruise somewhat, so what is it, and how can I get rid of it?