Twelve square beams make the frame of a cube, painted from the far side to the near one. Paint two of the edges that meet at the farthest corner last, and those far edges land in front of the near corner. Every edge stays straight and every corner stays square, yet the solid cannot be built. At the default angles, 45 and 35.3 degrees, the far corner and the near corner fall on exactly the same spot on screen, which is where the swap bites hardest.
The panel names at the top right open the cube, motion, and color panels. Cube sets the edge length, the thickness of the beams, the heading, the elevation, how many edges are brought to the front, and how much of the screen the figure fills. Swap none and you get a cube seen from the front; swap all three and you get the same cube seen from behind. Only one and two break it. Motion sets how far and how fast the figure sways, and whether it runs. Color sets the top face, the right face, the left face, the outline, and the ground.
Drag the stage to move the figure and use the wheel to zoom; the field at the bottom right takes a zoom value too. Double-click a panel title to switch how the panels are laid out. In the movable layout you drag panels wherever you like and keep several open at once; in the fixed layout a single panel sits in the top right.