This presentation focuses on mathematics tasks that challenge students, which have proven beneficial for learning. Tension (as an opposite to cohesion) between different modes (e.g., words, symbols) can create such challenge. By using cohesion and tension as concepts to analyse students’ work on multimodal tasks, this study aims to identify characteristics in the task design that makes tudents develop understanding of the mathematics concept in focus. Preliminary results show that a high degree of cohesion can direct the attention of students to the intended tension. Also, the tension in a task, although relevant, can be too challenging for students, thus ignored, and make students rely too much on previous knowledge.