Illusion is a good medium to create special effects in an image. The device of illusion is especially useful for photographing lifeless items to give the viewers a creative perspective to the otherwise boring feeling of stillness in the photo. "Water", the topic of this month's photo contest at ricohforum, is exactly when illusions can help bring the image to life. A clear example is Danielygo's entry.
You don't need expensive tools to achieve illusions in an image. Apart from making some effort to use our imagination as Danielygo did, we can make use of the illusions readily available.
Reflective surfaces are one of the approaches I use a lot. There is actually a tag titled "Reflections" for the posts here. As far as my experience goes, the effect is best when the surfaces reflect images from various directions and reproduce them with different opacity qualities. I have the penchant for even using reflection to give out this riddle: where is the photographer? Such an image is only possible when the reflective surfaces actually do not but seemingly do point to the position where I am shooting.
At first glance on the spot, the result may seem less obvious because the reflections seem not really illusive to the three-dimensional visual sense. But on a two dimensional image, the spatial factor becomes less obvious and so the reflections puzzle and interest the mind more greatly.
Another yet more handy way to achieve illusions in the image is to turn the picture upside down. I will revisit this topic when I have an obvious example.