Japanese Noh masks are used in an incredible form of traditional Japanese theatre believed to date back to around the 13th century. The wooden masks require a great deal of time and skill to craft.
The masks are truly an exceptional piece of art all on their own; however, in the hands of a Noh actor one can see their most amazing property. By rotating the mask about horizontal axis, the actor is able to convey various emotions to the audience. The mask apears to not be made of the rigid wood it has been carved out of but rather appears to change shape as it is rotated. When tilted up the mask looks sad but when tited down it conveys happiness.
The reason the mask is able to do convey emotions like this is due to exaggeration of facial features, especially the mouth and lips. To explain it quickly: as the mask is tilted up the upper lip appears to curve downward in a frown, when the mask is tilted down the upper lip appears to curve upward in a smile, and when the viewer sees the mask head on with no rotation the upper lip appears as a normal curved lip conveying no emotion (or if anything: indifference). Pretty much the same principle applies to the eyes as well.