I noticed a couple days ago that the Digital Crown on my Apple Watch wasn’t turning quite as easily as it had when I got it. Of course I immediately thought it was a defect since it visually looked clean. Turns out it was needing a bit of maintenance. 

I use a treadmill desk and even with the 3mph speed I sweat a bit. The Watch was designed to handle fitness scenarios but apparently it can get gunked up without some maintenance. Simply wiping down the exterior is not enough. 

Apple has realized this too. They even published a support document specifically about the Digital Crown and their recommended process to remedy it:

https://support.apple.com/en-us/ht204639

It was a little unnerving putting the Watch under running water, but it worked. Now you know.