Howard Blackwood explains that Memory Residue is left behind when something "ceases to be". He also suggests that, somehow, Memory Residue is necessary to the process of changing the past, specifically when concerning the Book of Memories. Memory Residue is also supposedly used to keep track of what occurs in the dungeons for the protagonist to remember the next day. In spite of this, saving the game does not cost the player any additional Memory Residue.
From the "On Memory" excerpt:
- Ever had a friend remember a shared event entirely differently than you remember it? When a memory is "deleted" from the human brain, it does not completely disappear. It leaves behind Memory Residue.
- From this "MR", new memories can be formed; not the usual way — by experiencing outside stimuli —, but by repurposing the MR into a new, changed memory.
If the player has the Expansion Pack, they can find Princess Heart weapons. They sell for 7,000 MR and are the most profitable weapon of the game, next to the Great Spear (4,000 MR), TV Remote (3,000 MR) and Television (2,250 MR).
Because the weapons respawn in zones when the player resets the game, they can essentially exploit selling them to farm MR:
- Save in library > Find Princess Heart(s) > Run to Howard's Shop and sell them > Save > Reset game (repeat)
This method is especially useful if the player finds two (or more) Princess Hearts in a zone (along with a Great Spear or TV Remote), meaning they can easily earn 14,000 MR every 10 minutes or so. For comparison, the most expensive item in the game is the Silver Bracer of Serpents (77,700 MR) and the second most expensive item is the Chamuel's Rose Quartz (27,000 MR).