Once, he made a bet with her that
he could measure the weight of smoke.

  
                   
You mean, weigh smoke?

  
                   
Exactly. Weigh smoke.

  
                   
You can't do that.
It's like weighing air.

  
                   
I admit it's strange. Almost
like weighing someone's soul.

  
                   
But Sir Walter was a clever guy.

  
                   
First, he took an unsmoked cigar and
put it on a balance and weighed it.

  
                   
Then he lit up and smoked the cigar,

  
                   
carefully tapping the
ashes into the balance pan.

  
                   
When he was finished, he
put the butt into the pan

  
                   
along with the ashes and
weighed what was there.

  
                   
Then he subtracted that number from the
original weight of the unsmoked cigar.

  
                   
The difference was the
weight of the smoke. 

(Excerpt from transcript of 1995 movie Smoke)