The retina, at the back of the eye, is covered with light-sensitive cells.
There are no light-sensitive cells where the optic nerve enters the eye and this means there is a part of your eye where you cannot see anything at all. This is called the blind spot.
There are lots of ways of exploring your blind spot. The simplest is to draw a small circle and a small square side by side on a piece of paper, about 10cm apart. Draw the circle on the left, and the square on the right.
Hold the paper at arm's length. Close your right eye and look at the square with the left. Slowly move the paper towards you.
At a certain distance, you will no longer be able to see the circle, as the reflected light from this part of the page is hitting your blind spot. If you continue to move the paper towards you, it will reappear.
You can try this with the other eye. Close your left eye and look at the circle with the right eye. Slowly bring the paper towards you and you will find that the square disappears.
You can see other effects of the blind spot - and how your brain adapts to fill in the missing information.
Draw a circle on a piece of paper. On the left side draw a thick line with a break in it half way along. The line should be about 10cm long, and the break 1cm. Close your right eye and look at the dot with the left eye.
Hold the paper at arm's length and slowly draw it towards you. Most of the time you will be aware of the broken line, but when the gap between the two lines falls on your blind spot you should find that your brain interprets the image as a continuous line.
The brain is compensating for the presence of the blind spot.
Sophie Duncan is project manager for science at the BBC www.bbc.co.ukscience