In its purest form, water is transparent, or at least clear. (Those two do not necessarily mean the same thing—look ‘em up!) So clear, in fact, that you can read through it, if so inclined. Most water, however, is not in its purest form, and many factors can affect how much under- or through-water visibility one experiences. One of these factors is turbidity.
Just Add Turbid
To demonstrate the effect of turbidity on water, you can use this easy-peasy-lemon-squeezy little Demo Science science demo. All you’ll need are two clear water receptacles (beakers or jars, glass is generally best), a bit of sand (more than a handful, but not much more), two pieces of paper, a writing utensil, and, of course, some water.
First, use your pen (or whatever) to write something on the paper—we recommend “turbidity,” if you wanna be all meta about it. Then, fill your receptacles with water, leaving a bit room at the top. Put one jar over each piece of paper so you can see the writing through the water.
Next, add your sand to one of the jars. Have students (or whoever it is you’re demonstrating this jazz to) look through first the clear-water jar and observe the visibility. Then, have them scope out the sandy-water jar and observe the same.
It doesn’t take a rocket surgeon to figure out that the water with the sand in it will be harder to read through. Unless you the sand somehow settled on the bottom in the exact shape of the letters in the word(s) you wrote, which would likely require some sort of magic, which would be pretty rad and would make science exceptionally boring by comparison.
Is There Actual Science Involved, or Am I Just Going Off On Tangents for the Rest of This Piece?
Science, yes there is the that. Turbidity is, as your students may have gathered, the degree of cloudiness of a fluid (it can also affect gases like oxygen—see Beijing, China for an example of that). As such, turbidity directly influences visibility.
Areas where water is in motion, like along beaches or in rivers, naturally create turbidity by mixing sediment and other particulates into the water itself. You can demonstrate this effect by waiting for the sand in your jar to settle to the bottom—have the kids look through the sides of the jar this time and observe the clarity. Then, swirl the jar around a bit to kick the sand back up and have them take another look.