http://chomaloma.blogspot.com/2011/02/social-network-inaccuracies-regarding.html//OS 教室里面的场景
Professor -- "One train is leaving City A at 100km/hr, towards City B. A second train leaves City B at.. anybody?"
Student -- "160km/hr!"
Professor -- "That is correct. Does everyone see how he got there?"
"???"