Number 4: The disappearing O.

Here I've got a really great and surprising example of how your brain makes stuff up.

What you'll need:
Nothing really, just this page. Or you could draw the X and O on a bit of blank paper.

          X                                                                    O                             

What to do:
1) Close your left eye.
2) Look at the X, but pay attention to the O.
3) Slowly move towards the screen.
4) When you are about 15cm or 6 inches from the screen (you'll need to be closer on a small screen) the O should disappear.

What's going on?
The retina at the back of your eyes is covering in cells that are sensitive to light. But at the spot where the optic nerve (which connects your eye to your brain) is attach to the retina there are none of these cells. Any light that falls on this blank patch isn't registered. Which means that when the light from the O hits that spot you don't see it.

So how come we don't notice this blind spot all the time? Well you're brain is pretty amazing, it just makes up some stuff based on what it sees around the blind spot and clones it in the space. There is white space around the O and thats what your brain sticks into the gap. You can see just what a good job your brain does by doing the same experiment on the X below.

Once again close your left eye and pay attention to the sentence 'Nothing missing is there?'. Nothing missing is there?

X                                                 Nothing missing is there?