I'm rather fond of the many DIY engineering hobbies that some people focus extremely deeply on. Air cannons were a big interest of mine back in 2009 or so: http

I'm rather fond of the many DIY engineering hobbies that some people focus extremely deeply on. Air cannons were a big interest of mine back in 2009 or so: https://www.spudfiles.com/ https://www.spudfiles.com/ Someone put a (rather primitive) air cannon simulator I wrote on GitHub: https://github.com/cli-mongrel/bags https://github.com/cli-mongrel/bags And before then I was in to homemade water guns, e.g.: http://www.sscentral.org/homemade/supercannon2.html http://www.sscentral.org/homemade/supercannon2.html I built that when I was a teenager as I recall. Another example that I never participated in: http://www.pulse-jets.com/phpbb3/index.php http://www.pulse-jets.com/phpbb3/index.php I'd love to get better at machining so I could make some small jet engines like those folks. I'm similarly impressed by people who modify cars, build wind turbines, etc., etc. If there are any particular hobbies along these lines that you find interesting, I'd love to hear about them!