A gut check is a rapid intuitive evaluation of a design concept, prototype, or decision, a deliberately fast, unanalysed first response from a person representative of the target user or a domain expert. In design practice, gut checks complement analytical evaluation by capturing the immediate, pre-rational reaction that is often the truest indicator of whether something feels right or wrong. The method acknowledges that first impressions in user experience are both powerful and irreversible, the first five seconds of exposure to a design produce reactions that matter independently of any subsequent analysis.