Learning mathematics is often justified as supporting students to make rational decisions, an important goal given the misinformation spread through social media every day. However, when asking preservice teachers to engage in a critical mathematics education activity in which they had to produce an argumentation that would persuade people to change their behaviour using mathematics, we found that the need for a truthful argument was replaced by the need to make an impact on their audience. We use Frankfurt's distinction between truth-tellers, liars and bullshitters to discuss how this change took place and what implications there are for teacher education.