How can you keep straight face in situation like this?

I was at work last friday and two customers came who i found them very good looking. They were talking to colleague and they were asked to wait. While they were waiting one of them came up to me and started asking me where are you from? and you look from spain whi ch i said Im not spanish. He spoke few more minutes and he sat down right in front of my table and kept on stairing me. At the start i ddidn'tbother but it went on for long time and i got really nervous. I know i shouldn't give s.h.i.t but i couldn't concentrate on my work and he ddidn'tturn away. So in situations like that how do u keep straight face?