Pentachoron

http://en.wikipedia.org/wiki/Pentachoron

Regular pentachoron, Schlegel diagram (5-cell) (4-simplex)

In geometry, the pentachoron is a four-dimensional object bounded by 5 tetrahedral cells. It is also known as the 5-cell, pentatope, or hyperpyramid. It is a 4-simplex, the simplest possible convex regular 4-polytope (four-dimensional analogue of a polyhedron), and is analogous to the tetrahedron in three dimensions and the triangle in two dimensions.
The regular pentachoron is bounded by regular tetrahedra, and is one of the six regular convex polychora, represented by Schläfli symbol {3,3,3}.
The pentachoron is self-dual, and its vertex figure is a tetrahedron. Its maximal intersection with 3-dimensional space is the triangular prism.

A 3D projection of a 5-cell performing a double rotation about two orthogonal planes

[Source: en.wikipedia.org]

Same Colour Illusion

http://antwrp.gsfc.nasa.gov/apod/
[This, from the APOD on the sidebar...]

Are square A and B the same colour? They are. Are too. To verify this, click here to see them connected. The above illusion, called the same colour illusion, illustrates that purely human observations in science may be ambiguous or inaccurate. Even such a seemingly direct perception as relative color. Similar illusions exist on the sky, such as the size of the Moon near the horizon, or the apparent shapes of astronomical objects. The advent of automated, reproducible, measuring devices such as CCDs have made science in general and astronomy in particular less prone to, but not free of, human-biased illusions.

[Source: antwrp.gsfc.nasa.gov]

Seen on the F-ring of Saturn...