What reaction do you have to this picture of the god Bachus?
What sense you make of what you are looking at depends on how you are "primed"
Look again. There is no old man - it's a young couple kissing under an ivy covered arch.
Of course, some people may be primed by personality so they always see the young couple first. Once you have seen both possibilities, you have the option to choose which you prefer.
What you see also depends on what you are paying attention to
If you want to watch some videos which illustrate this, try following these:
Change blindness experiment (New version of famous experiment)
Change awareness test (UK road safety advert)
The two key points to take away - and use to solve the toughest problems - are:
1. Our perception is always an interpretation - we see a subjective NOT objective reality
2. What we see (or don't see) creates the pattern for further interaction - it's self-fulfilling