Origins of "Spend a Penny"

During our walk to the Crystal Palace with Tracy and Griff, I heard them use the phrase "spend a penny" which I had never heard.  It turns out to be a euphemism for "going to the bathroom"; its origins were right there at the Crystal Palace itself.  When it opened in 1851, they debuted the use of pay toilets the cost was one penny.  Now they have pay toilets in London that are 30p (80 cents) but the phrase still is in use here.  "Excuse me, but I'm going to spend a penny."  This is considered a more polite was of the other euphemism "going to the loo" which is viewed as slightly vulgar.