Yeah yeah, we know Doom runs on everything from calculators to ATMs, but that's Doom. There's something about seeing Half-Life running somewhere crazy — like some guy's watch — that makes it more impressive.
Using the SDLash app, you can boot and play Half-Life (or, I guess, any older game using the Source engine) on an Android Wear watch. While it's far from perfect — the game is hard to control and crashes — that doesn't change the fact you can play Half-Life from your watch.
Note that this is an original LG G Watch; bigger Wear devices, at least those with square screens, might make things a little easier.