One of the first things we notice is that, while the battery dominates the interior space of the watch, it barely shows up in X-ray images. (It's the sort of smoky-looking patch on the right.)

Like everything in the Apple Watch, the battery was designed to be small and super thin. In larger devices such as smartphones, the thicker batteries absorb more X-rays, and thus are easier to spot.

The densest (and therefore darkest) components in the picture are the magnets—such as in the speakers on the lower left, the Taptic Engine, and the small magnet in the center that aligns the charger.