Have you ever learned a new word or noticed a particular car model for the first time, only to suddenly start noticing it seemingly everywhere over the following days? This common experience, sometimes referred to as the frequency illusion, is not evidence of any actual increase in the frequency of the word or object itself, but rather a cognitive bias involving two closely related psychological processes working together. The first is selective attention: once something enters conscious awareness, the brain becomes considerably more likely to notice subsequent occurrences of that same thing, information that would previously have simply been filtered out as irrelevant background noise. The second contributing process is confirmation bias, in which each new noticed occurrence appears to further validate the initial impression that the object or word has become suddenly and objectively more common, even though its actual real-world frequency has remained entirely unchanged throughout. This phenomenon reveals something genuinely important about the nature of human perception more broadly: much of what we perceive as reality is actively and selectively filtered by attention, meaning our subjective sense of how common or rare something is can diverge substantially from its objective, measurable frequency in the world around us.