Users continue to stumble upon software bugs, despite developers' efforts to build and test high-quality software. Although traditional testing and quality assurance techniques are extremely valuable, software testing should pay more attention to exploration. Exploration can directly apply knowledge and learning to the core of industrial software testing, revealing more relevant bugs earlier. This article describes exploration's characteristics, knowledge's role in software testing, and the three levels of exploratory-testing practices. Academics and practitioners should focus on exploiting exploration's strengths in software testing and on reporting existing practices and benefits in different academic and industrial contexts.