The easiest way to demonstrate this is to simplify the subject. Try visualizing a simple geometric figure, a red triangle. The triangle will blur, twist, warp, crack, squiggle, darken, and lighten. In short, do something which no self-respecting simple geometric shape ought to do. You might think that the experiment was different. In one case, you are "concentrating on a subject," and in the second case, "visualizing" a subject. However, the experiment, attempting to focus the mind, is the same, the problem is just more obvious in the second case.
To test yourself, keep a notebook and clock handy and maintain a record of each time your mind wanders from visualizing the red triangle. With practice, you will be able to concentrate for a longer period of time, and the "breaks" will decrease. You will note that the breaks can be classified as six types. The first type of break is physical sensations. The second type of break is when the mind wanders to recall events which occurred immediately before your meditation. These are often matters that you did not even consciously notice. The third type of break is daydreams. These are particularly problematic in that one can become lost in a daydream without realizing that one is daydreaming. The fourth type of break is when one stops concentrating on the red triangle and, instead, begins to think about the act of concentrating on the red triangle. This is not at all the same thing, any more so than discussing dinner satisfies the hunger. The fifth type of break consists of actual auditory hallucinations, the hearing of strange phrases or sentence fragments in an unrecognizable voice. These are usually harmless, although they can be quite disconcerting. The sixth type of break, unlike the other five, is desirable, and is called dhyana.