Observe the inmate with the prison map painted over on his back.
Ignore the standard cleaning items like the washcloth, toothpick, soap, butter, and toothbrush, as they will all fail to reveal the map.
Locate the mouse hole in the wall near the sink.
Drag the mouse from the hole to the inmate's back to have it lick away some of the paint.
Finally, interact with the other inmate (the cellmate) to have him lick off the remaining paint with his long tongue.
Once the map is fully revealed, the guard will arrive to release you, completing the level.
Level 21: Notes and Precautions
Ineffective Items: Do not waste time using the washcloth, soap, or toothbrush; the game specifically designs these to fail to emphasize the unconventional solution.
Sequence: You may need to use the mouse before the cellmate can finish the job.
Visual Cues: The red "X" on the bible and the cracks in the wall are mostly decorative and do not directly help in cleaning the map.
Level 21: Tips and Tricks
Think Outside the Box: In this game, "cleaning" often involves non-traditional methods. If soap doesn't work, look for something that might "eat" or "lick" the substance away.
Interactive Environment: Always check for small details like the mouse hole or the cellmate's unusual features (like a long tongue) to find the solution.
Cellmate's Role: The caption "Let our cellmate do the work" is a hint to look for living creatures in the cell to help you.