Guitar Testing

I picked up my friends Roger and Nha from JFK today after their 20 hours of flying from Sydney, Australia.  We had a lovely dinner and sat around in our back room visiting.  When they went upstairs, Roger spied Amy's guitar case in the upstairs hall.  He immediately took the guitar out and started tuning it.  the first thing he said was that the guitar strings were old.  "How can you tell?" I asked.  Roger said that guitar strings rust, and then your fingers don't slide on them easily.  He got it a bit more in tune and began playing.  It was really wonderful to hear beautiful music coming from Amy's old guitar.