https://math.stackexchange.com/questions/3541631/snake-in-a-5x5-grid

Langley's problem -

https://www.mathpages.com/home/kmath277/kmath277.htm

https://samsmathclub.com/Forum/index.php?action=recent

https://en.wikipedia.org/wiki/Monty_Hall_problem

http://www.math.cornell.edu/~mec/2008-2009/TianyiZheng/Conditional.html

http://mathforum.org/dr.math/faq/faq.monty.hall.html

https://www.scientificamerican.com/article/the-3-door-monty-hall-problem/

monty hall problem

knowledge is very asymmetrical - if you don't know it, it's really hard; if you know it, it's very easy.

nature of STEM