Psychological essentialism is the idea that certain categories, such as "lion" or "female", have an underlying reality that cannot be observed directly. Where does this idea come from? This article reviews recent evidence suggesting that psychological essentialism is an early cognitive bias. Young children look beyond the obvious in many converging ways: when learning words, generalizing knowledge to new category members, reasoning about the insides of things, contemplating the role of nature versus nurture, and constructing causal explanations. These findings argue against the standard view of children as concrete thinkers, instead claiming that children have an early tendency to search for hidden, non-obvious features.