If you stare at the following picture long enough,
you should see a giraffe...
What are "illusions"? Illusions trick us into perceiving something differently than it actually exists, so what we see does not correspond to physical reality. Hence, the word illusion comes from the Latin verb illudere meaning, "to mock." In addition, some illusions show us one thing in a picture, while someone else sees something entirely different in the same picture.
Research scientists must be sure that the results of their work are not "illusory" in nature. They need to accurately report what "is", rather than their general "impression" of "what is". So many times a scientist will repeat an experiment many times, or in different laboratories, to ensure that their results were valid. Science is only "good science" when anyone can repeat the experiment and get the same results.
If your eyes follow the movement of the rotating pink dot,
the dots will remain only one color, pink.
However if you stare at the black " +" in the center, the moving dots turns to green.
Now, concentrate on the black " + " in the center of the picture. After a short period, all the pink dots will slowly disappear, and you will only see only a single green dot rotating.
It's amazing how our brain works. There really is no green dot, and the pink ones really don't disappear. This should be proof enough, we don't always see what we think we see.