The other day I learned about a method how to calculate a (pseudo)
random number in my head, given a not-so-randoom number you just
made up (the former is better).
  Last night I couldn't sleep and instead of countinh sheep, I
generate random numbers until... I found a random seed which always
returned 9! That seed would be 59, and yes, that kept me awake.
  Daaaang.
  More here
gopher://gopher.ynfonatic.de/1/~alex/2025-09-14-marsaglia-prng