Goddess Wall

A Goddess Wall is a sacred image that is featured in Skyward Sword. It can be drawn on with Link's sword after performing a Skyward Strike to receive various items.

Appearance
Goddess Walls are largely invisible and indistinguishable from other walls except for the fact that Blessed Butterflies congregate around them, as they do with buried Gossip Stones (though in the former case it will be fairly obvious they are indicating the wall as opposed to an open patch of ground).

When Link plays the Goddess Harp near one it will become visible as a blue/purple rectangular crest with the symbol of Hylia in the center.

Once Link's sword is energized he is able to draw on the wall by holding down A and drawing with the Wii remote.

Features
So far the known drawing options are a heart, an elongated hexagon representing a rupee, a circle representing a bomb flower, an arrow and the Triforce, which results in a gift of fairies.

Depending on how well Link draws the gift will become larger (you can get anywhere from 3 to nearly 30 hearts, several blue rupees and a bunch of green rupees or several red, blue and green rupees, etc).

The arrow in particular is not terribly straight forward. A picture of the shape necessary to draw to receive arrows can be visualized if Link visits a Sheikah Stone after a certain point in the game. The drawing consists of a single unbroken line starting from the left of the Goddess Wall and going to the right. At the right end, a triangle is formed by drawing up and to the left a short ways at a 45 degree angle, drawing down orthogonal to the original left-to-right line and finally drawing up and to the right at a 45 degree angle completing the triangle at the tip of the arrow.