Ask HN: How do you keep your mind sharp?

[squiguy7]

Since HN is a community of individuals who mostly like to keep learning, how do you maintain that discipline? After graduating a few years ago, I feel as if I forgot all of the math I studied and know it would be more difficult to learn it again. What are some of the best ways you have found that help you stay fresh?

[dnh44]

I don’t do much programming these days so I started working through the problems on Project Euler.

https://projecteuler.net

[oriel]

Do you know of any resources that discusses the mathematics behind the problems in depth? I've found a mishmash but nothing official or consistently well reasoned/discussed.