If you stare at the following picture long enough, you should see a giraffe...
Gotcha! But the rest of these optical illusions are on the level.
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.