When this page loads, it waits for 5 seconds, generates a random number, then generates a load of divs of random colour.
Go to another page. If your browser supports back/forward caching you should be able to click back and return to this page without a 5 second wait. The random number will not be regenerated and the divs will be the same colour they were before.
However, if you add an unload listener before leaving the page, the feature is killed.