This paper presents an exploratory study in which a humanoid robot (MecWilly) acted as a partner to preschool children, helping them to learn English words. In order to use the Socio-Cognitive Conflict paradigm to induce the knowledge acquisition process, we designed a playful activity in which children worked in pairs with another child or with the humanoid robot on a word-picture association task involving fruit and vegetables. The analysis of the two experimental conditions (child-child and child-robot) demonstrates the effectiveness of Socio-Cognitive Conflict in improving the children's learning of English. Furthermore, the analysis of children's performances as reported in this study appears to highlight the potential use of humanoid robots in the acquisition of English by young children.