Run your mouse over the the buttons.. Two have been have already been loaded and the other two haven't, so two will change quickly while the other two will have a small delay.

Right below this text I have two images hidden from site.. Can you see them?? Look at the sourse code.. They are there!!

All I did was change the dimensions to Width="1" Height="1". When you visit a webpage, the images on the page are being stored in your cache. When you are surfing through a site that uses the same background set through out the site, have you noticed the second page you go to always loads quicker than it did the first time? Those images have already been stored in your cache.
Here is one more example of this.. If you click here, you will see an image that is roughly 53k.. Now normally that would take a little time to load.. But as you have been reading this the image has already been loaded and is in your cache.
Keep in mind, if you are connected to the internet by a Cable modem or DSL, you may not be able to see a difference because pages load quicker anyways..

Hope this helps.. Like I said, though, this was a tip that someone else gave me and now I use it all the time.. It works..*S*