This study investigated the relationship between a metaphor-based approach to teaching English as a foreign language (EFL) and involvement of the brain's right hemisphere. Specifically, it examined learners' understanding of three levels of sureness associated with different expressions in English -- those that are "certain," "probable," and "possible" items. The three target items were chosen because they are frequently used by native English speakers although Japanese EFL learners often fail to distinguish between them. The metaphor-based learners and the control group engaged in computer-based explicit learning based on the meaning of the target expressions with three-dimensional (3D) animated illustrations. The images were based on the spatial concept of distance for the metaphor-based learners and a list of target items for the control group. At post-test, it was found that the metaphor-based learners performed better than the control group in both comparison and speaking tests. Obviously, the participants better understood the degrees of certainty in relation to distance. This shows that teaching the degrees of certainty by applying the spatial concept of distance could help in second-language (L2) learning. The metaphor approach allowed participants to link the spatial concept of distance to the degree of certainty associated with expressions of certitude. In addition, recordings of lip movements showed that participants remembered the target items better by opening the left side of their mouths more than the right side. This indicates that the brain's right hemisphere is involved in deep processing of expressions that reflect different levels of certitude and creating stronger memory traces.