Below is one of the toughest mind teasers I've seen in some time. Check it out.
On the link, you'll find two pictures that are exactly the same -- or so they seem. There are actually three differences and if you can find all three then you're lucky; a group of scientists claim you are in the top .01 percent of the most perceptive people on this great earth. I'm actually proud that I found one of them. I believe sound plays a role in this as well, so make sure it's on. Also, it takes a huge amount of concentration and is surprisingly taxing. The scientists actually counsel those with heart conditions to not even attempt it.
Good luck!