The answer to this is Sand. If you are confused how this answer is reached, you first have to look at the various choices that are available. What can be clear about this choices? All of them are solid objects but another thing to note is that almost all of them can be counted easily.
You can take a look at the pillows that you have at home and count how many that you have. There is one thing that you cannot count and that is sand. No matter how hard you try to count sand, they are too fine to be counted. Some would rather count and collect stones because this can be easier to accomplish.