Having a challenge; learning something new; struggling with a problem / new language and eventually getting to the point where you can write in the language like you'd speak in your native spoken language.
Been doing this for the last week or so - Enough people have asked for and I finally got sick of hearing it - an iPhone app for the site. So last week, I started writing it in Swift 3. I'm no pro yet and this won't be my first app in the store - but Swift 3 is new to me; I haven't had an app in the store in 3 years (last app was in Objective-C) and I'm starting to be able to write code from memory and not from reference. Or last night - the concept of auto layout finally clicked and I got that working.