What's the story behind?
The one who stopped Laughing was the smartest one among! Read how he was the smartest.
We need to think from the smartest Philosopher's point of view. Let's name 2 other Philosophers as A & B.
Now here is what the Smartest Philosopher would think.
"If I had nothing on my head then A & B must have been laughing after looking each other's head. And at least one of them is smart enough to realize that the other is laughing only after looking at him. That means A (or B) would have realized that the some thing is on his head too as B (or A) is laughing after looking him (not me if I had nothing on my head). Hence one of them would have stopped laughing. Since they are not stopping to laugh, I too must have something on my head."
Hence the smartest Philosopher Stopped Laughing after realizing that the fact.
A brilliant puzzle based on the similar logic is here!