Change blindness is a great way to demonstrate sensory memory. Sensory memory is very short-lived—less than a second for images—and most of the information it takes in doesn't enter our conscious awareness.
When we see two slightly different images, one right after the other, it is very easy to pick out the difference. But adding a 0.6-second flash in between makes the task much harder. That quick flash is long enough for most of the information in our visual sensory memory to fade. It's only when we intentionally focus our attention on specific visual elements that we can see the change.
To learn more about sensory memory, working memory, and more, visit Types of Memory.