So what exactly are we looking at in this Cliff Face Illusion. Is it one face or two faces and are they real or has someone been photoshopping? We would love to hear your thoughts.

The best optical illusions are often the ones we happen upon unintentionally, which is exactly what happened when redditor Liammm decided that water circling the drain of his sink would make for a nice photographic subject.

This jigsaw includes pieces of lots of different animals, but one stands out more than all the others. To see it you will need to stand well back from the monitor. We understand that the original artwork was produced by Donald Rust.

We have looked at this picture a number of times and are still not sure which building is in front of the other. Any idea?

It’s hard to see but these wavy strings are actually perfect concentric circles.

This is an old favorite, how many skittles do you see? You might count from the top and say - nine, but then count from the base. What's going on!!!

Even the great Isambard Kingdom Brunel can have an off day. Which it looks like he did, when he designed this bridge in Somerset, England. Not really, just a little tweaking by us.

This is Optical Spy's office bicycle. What we can't work out is if the wheels are spinning or not.

M.C. Escher printed Metamorphosis I in 1937, which was a beginning part of a series of designs that told a story through the use of pictures. These works demonstrated a culmination of Escher’s skills to incorporate mathematics into art. In Metamorphosis I, he transformed convex polygons into regular patterns in a plane to form a human motif. This effect symbolizes his change of interest from landscape and nature to regular division of a plane.

One of his most notable works is the piece Metamorphosis III (above), which is wide enough to cover all the walls in a room, and then loop back onto itself.

Everybody knows how great it would be to have a swimming pool in their back yard. But, the costs of cleaning and filtering are so prohibitive it puts off all but the very rich. So here at optical spy we decided to invent a pool that needs hardly any maintenance - The Self Filtering Swimming Pool. What do you think of it?

The craters were discovered on the southern hemisphere of Mercury, northwest of a larger crater known as Magritte. The largest crater making up Mickey’s ‘head’ measures at around 65 miles (105 kilometers) across. The photo was taken during NASA’s Messenger probe, which aims to collect images when the sun is near the horizon. During these times, the sun produces long shadows that bring up smaller features on the planet’s surface.

Trompe-l’œil, which can also be spelled without the hyphen in English as trompe l’oeil, (French for deceive the eye) is an art technique involving extremely realistic imagery in order to create the optical illusion that the depicted objects exist in three dimensions. Forced perspective is a comparable illusion in architecture.

Although the phrase has its origin in the Baroque period, when it refers to perspectival illusionism, use of trompe-l’œil dates back much further. It was (and is) often employed in murals. Instances from Greek and Roman times are known, for instance in Pompeii. A typical trompe-l’œil mural might depict a window, door, or hallway, intended to suggest a larger room.

A version of an oft-told ancient Greek story concerns a contest between two renowned painters. Zeuxis (born around 464 BC) produced a still life painting so convincing, that birds flew down from the sky to peck at the painted grapes. In order to judge his rival Parrhasius’s painting, Parrhasius asked Zeuxis to pull back a pair of very tattered curtains in his study. When Zeuxis tried, he was unable to do so as the curtains were Parrhasius’s painting, making Parrhasius the winner.

With the superior understanding of perspective drawing achieved in the Renaissance, Italian painters of the late Quattrocento such as Andrea Mantegna (1431–1506) and Melozzo da Forlì (1438–1494), began painting illusionistic ceiling paintings, generally in fresco, that employed perspective and techniques such as foreshortening in order to give the impression of greater space to the viewer below. This type of trompe l’œil illusionism as specifically applied to ceiling paintings is known as di sotto in sù, meaning from below, upward in Italian. The elements above the viewer are rendered as if viewed from true vanishing point perspective. Well-known examples are the Camera degli Sposi in Mantua and Antonio da Correggio’s (1489–1534) Assumption of the Virgin in the Duomo of Parma. Similarly, Vittorio Carpaccio (1460–1525) and Jacopo de’ Barbari (c.1440–before 1516) added small trompe-l’œil features to their paintings, playfully exploring the boundary between image and reality. For example, a fly might appear to be sitting on the painting’s frame, or a curtain might appear to partly conceal the painting, a piece of paper might appear to be attached to a board, or a person might appear to be climbing out of the painting altogether—all in reference to the contest of Zeuxis and Parrhasius. In a 1964 seminar, the psychoanalyst and theorist Jacques Lacan (1901–1981) observed that the myth of the two painters reveals an interesting aspect of human cognition. While animals are attracted to superficial appearances, humans are enticed by the idea of that which is hidden.

Take one look at the beautiful and detailed sand drawing of an elephant and the impulse is to believe it was created by one incredibly talented and meticulous artist. But at second glance, the drawing looks perfect, almost too perfect, to be the work of human hands. That’s because the artist doesn’t have hands, in fact she has four legs, weighs over two tons, and created this masterpiece while sound asleep. This is a self portrait.

The drawing is really an imprint, made by Sundara, an eight-year-old Asian elephant who fell asleep at the Chester Zoo in England, leaving an impression so detailed that it is possible to make out the folds in her ears and the ridges in her trunk. Sundara made the imprint during what must have been a deep, almost motionless slumber, something which is highly unusual for an elephant, which generally has to get up in the middle night to switch sides due to its weight, something which in this case contributed to the immaculate detail of the impression.