Also, I'll add a small psychological explanation: Why are people seeing things in rocks?
Two things actually cause this, first
Pattern recognition which by now is hard-wired into our brains. Without this almost no creature will be able to stay alive, after all it's survival of the fittest out there. Lets use a very old human in a very hostile environment as an example.
A human with a prominent brow ridge and uncomplicated piece of fur garment covering his torso is standing in the middle of a thick forest. He peers around as the moon provides ample light to guide him through the forest. He knows this is no friendly area (especially at night) and his senses are heightened. His hand clutches a sharpened stick, ready to strike at any moment. He reaches an open spot in the forest and looks around; he spots something in the corner of his eye and quickly turns his head. The moon illuminates a golden entity with char black spots, two jade-colored menacing eyes stare at him, piercing his soul. His adrenal glands release their hormone and his heart goes into over-drive while dilated pupils give him a clearer vision, time almost seems to grind to a halt. Before his mind knows what is going on, his arm goes up and back and with immense force he throws his sharpened stick, striking the entity. The human and entity scream in unison, one in victory whilst the other in terror and pain. The dark entity drops from its branch and the moonlight reveals its feline characteristics. The once golden colour changes into a red shade whilst thick blood gushes from its neck. The human quickly paces at the feline creature, retrieves his weapon and quickly gives the creature the final blow which is instantly followed by a bloodcurdling shriek. Silence. The human slowly comes to realization what has happened as his mind catches up with the events. He looks down to see his hands red with blood, at his feet lies the liveless body of what his tribe calls a Warcat. After cleaning his bloodstained hands with some leaves, he firmly grasps his thrustworthy stick and moves onwards into the night.
If he didnt recognize the pattern of spotting a jaguar he would have most likely reacted too late and the jaguar would have got him. Pattern recognition is needed. Pattern recognition keeps us alive (not just in the old days), think of spotting a car-like object in the corner of your eye as you are about to cross the street. That split-second of seeing a car-like object will keep you from blindly crossing the road and in that case, probably keep you alive.
Second thing which in retrospect influences what you see and what you want to see is called
Confirmation bias. Basically if something doesn't fit in your world view or you don't want it to fit in.. your mind will abide that. It will keep all the non-fitting ideas out and almost no matter what it will keep it that way. Just read the wiki article, it's pretty interesting and also explains why religious folk will never accept science. Really, read the wiki article. Think about what you just read and think about religion and religious people.
Big PS:
What did you guys think of the "lil story" I wrote?
It took longer than I expected it would.. but hey, I actually liked writing like that. It was fun and creative.
Honestly, let me know what you think. Cheers.