Sight.
It is one of the senses that we are most reliant on to gather the information that we that we use to navigate the world around us. The spherical sacs of fluid embedded in our skulls (eyes) that give us a picture of the world, are not perfect, however. A large percentage of the human population has some form of low visual acuity (technical term for clarity of vision). We usually value our vision so much, that we are quick to correct the imperfection, with glasses, or contact lenses. But what if you don't have those with you right now, and you want to see something clearly?

Here's a little trick that will let you do exactly that.
Take off your glasses (if you have a pair) or just look at something at a distance. Look at anything that looks a little blurry.
Now take your hand, and form a tiny little hole, like this and put it up to your eye. Do you see the difference?

This video will explain exactly how to make it, and a little bit about how it does.

Let's understand a bit more about the effect.
It is called the pinhole effect. It got its name from its usage in old cameras that didn't have lenses to focus the light onto the film. To ensure they had a clear picture, they used a hole in a board made with a pin (hence, "pinhole") as the aperture. The advantage of doing this was that they didn't need to worry about focusing on a single spot anymore.
The first of these cameras were made as far back as the 10th century AD, and the technology has developed ever since to take long exposure pictures of objects. It's utility for us lies in obtaining a clear image of an object that was not in the focal plane (technical term for everything at the same distance from the eye as the focus). This allows pinhole cameras to have an infinite depth of field (another technical term that tells you the size of the focal plane), which means everything, near and far, seems clear and focused.

By allowing the light to pass through only a single point, this prevents blurriness and makes the image clearer (in an eye or a camera).
This is also why some people squint when they can't see clearly. The squinting reduces the size of the aperture (technical term for the size of the pinhole) and this makes objects less blurry.

The disadvantage is, though, that since you are blocking light rather than redirecting it, you lose a bunch of it, and your image is darker.

Lens images have more light, so are brighter than pinhole images.

So now you may want to build your own pinhole camera, and here are some helpful intructions on how to do just that.

And lastly, today (27th April, 2014) is actually World Pinhole Day. Congratulations!

Here are some more images of pinhole photography, along with information about World Pinhole Day. (http://www.bbc.com/news/in-pictures-22150973)