(Pretty old art but I still like it)
The brain is thought to have an unlimited amount of space to store our knowledge and experiences; and yet, we can lose so much information over time. We can lose it in our short term memory if we choose not to repeat it in our minds, and yet, that isn't always the case. For example, there can be songs you despise yet it sticks in your brain. Or, a grotesque image that's been imprinted in your mind that you can't get rid of.
The subject of memory is vastly studied in cognitive psychology and neurology, because there is so much to explore. How does the subconscious mind work? Is there really a limit to our brian storage?
My own memory is heavily temperamental. I can be horribly forgetful at the best of times. I can recall something from my childhood, yet I can't remember what I did yesterday.
The idea for this came from 'decay theory'. Which is basically, the bits of information we lose in our short term memory. I had the image of those bits of 'decay' information floating away from the brain and disintegrating.