Read the sign. What does it say? Are you sure? Our brains make associations with well known phrases, and will negate what we don?t think is there.
Actually, the sign reads ?I love Paris in the the springtime?. Because we are familiar with this famous phrase, we simply do not see the extra ?the?.
This optical illusion works in the same way as counting the Fs, where we forget to count ?OF?.
Our eyes see the words, but our brains interpret what we see, sometimes mistakenly! The red triangle also takes our attention away from each individual word written here.