I just discovered that a cool experiment, which was devised in 1999, can be done on the web. AND you can take part. Your results may end up surprising you a lot. In fact, if you haven't heard about it, you'll be VERY surprised at what most people's results are.
Unfortunately, I can't say what it is about without spoiling the whole thing, but it is easy and harmless. In the comments, I can say a bit about what it's taken to show, though I certainly am not a vision scientist. As for more general implications about whether we do understand ourselves very well, your opinion is probably more interesting than mine!
BTW, I give the DVD form of the test to my students; the students who do what you might think is "failing" tend in general to have higher grades. But that's for the discussion.
Here are the instructions:
When you follow the link below, a new window will open and you will see a short video. In the video two groups of people are passing a basket ball to each other. You should focus your attention on the people in white t-shirts. Try to count how many times the ball is passed between them during the video. This can be a bit tricky as the other group, in black t-shirts, must be ignored. ... Watch the video only once.
THE VIDEO IS JAVA; IT MAY TAKE TIME to download. And you'll need java enabled on your browser.
Here's the link to the video.
After you've watched the video ONCE, you probably will find it helpful to go to the site from which I drew the instructions above. That's here.
Please let us know if you were surprised after you've seen the explanation on the second site.