Brain teaser # 28

Prateek Agarwal (Sr. Executive - Finance & Accounts)   (1732 Points)

12 July 2012  

 

Two sentries were on duty outside a barracks. One faced up the road to watch for anyone approaching from the North. The other looked down the road to see if anyone was approached from the South. Suddenly one of them said to the other, "Why are you smiling?" 

How did he know that his companion was smiling?
indecision