It is not obvious...at least not to me...

When I was a tour guide in Greece two guests (a couple) looked super happy and said to me:

- Look look and they showed me their hands and they had really super nice watches.
I said: -Yeah really nice watches. They looked surprised and laughed and replied: - No look again.
I looked again and I did not see anything. -They said we got engaged.
Ops yet another embarrassing moment in my life.