Absolute Johannes factotum
- SCP-4430 (?)
- SCP-4430 refers to copies of a children's book titled "The Faceless Live In Evanholly," which, despite appearing harmless, causes a small number of children between the ages of three and seven to vanish without a trace. Parents often don't remember buying the book, and while the book itself doesn't seem to have any strange effects in a lab, children who read it sometimes go missing, with the book's story suggesting that a character named Masquerade removes children's faces so they can live in a place called Evanholly. The Foundation contains it by routinely checking homes with young children for copies of the book and seizing them, though the exact way it causes disappearances is still unknown.
- Tarski's circle-squaring problem (?)
- Tarski's circle-squaring problem, posed in 1925, asks if a disc can be cut into a finite number of pieces and reassembled to form a square of equal area. This seemingly impossible task was proven solvable by Miklós Laczkovich in 1990, though it requires using the axiom of choice, leading to non-constructive, non-measurable pieces. Later, constructive solutions using Borel sets were also found. However, it's impossible to achieve this if the pieces must have boundaries that can be cut with an idealized pair of scissors (Jordan curves). The problem is distinct from the ancient "squaring the circle" problem, which involved constructing a square of equal area using only a straightedge and compass.