Many people at one time or another have experienced déjà vu. French for "already seen," déjà vu is a sudden strong feeling that a moment identical to the present one has occurred at some earlier time.
To a cognitive psychologist, déjà vu is proof of the immense amount of knowledge and experience we store in our brains. When we experience déjà vu, what actually happens is that, in a fraction of a second, we retrieve bits of many different memory fragments and piece them together, producing what seems to be a complete memory.
I've Been Here Before...
So, if you experience déjà vu in a mall restaurant while waiting for a pepperoni pizza with your best friend, your mind has taken perhaps hundreds of stored memories of various experiences, and put together fragments from those memories to give you the sensation of having been there before, even though you haven't been there before at all.
Cognitive psychologists who study how we use language are not surprised at the brain's ability to create déjà vu. Actually, language comprehension and déjà vu have many parallels.
When you hear someone speak, you usually understand them even though you've probably never heard their words presented in exactly the same way. You understand these sentences because your brain is able to remember the individual meanings of words, based on hundreds of past experiences with those words.
Your brain takes the meanings of individual words and splices them together to comprehend their meaning as a whole. As with déjà vu, this entire process happens in a split second.