Why 18 Hole of Golf?

Why do full-length golf courses have 18 holes, and not 20, or 10 or an even dozen?  How many of you golfers out there really know the answer to this one?

During a discussion among the club's membership board at St. Andrews in 1858, one of the members pointed out that it takes exactly 18 shots to polish off a fifth of Scotch.  

By limiting himself to only one shot of Scotch per hole, the Scot figured a round of golf was finished when the Scotch ran out.