The viewer stands on one side of the illusion and the skeleton or monster (or a second viewer) stands on the other side. There is a two-way mirror suspended in the enclosure, spaced evenly in the middle.

There are lights on both sides of the enclosure. When the lights are on the viewer's side, they see a reflection of their own face. When the lights are on the skeleton's side, the viewer can see through the mirror and catch a glimpse of the skeleton.

If the viewer aligns their eyes with the skeleton face, the motion of the lights will appear to blend the two faces together, transforming one into the other and back again. This is spooky with a skeleton transformation -- the flesh appears to melt away into bone and then back again. It's also very effective with a second human on the back side -- the faces melt into each other, creating a visage that's a blend of the two faces.