Introducing measurement uncertainties aims to foster middle school students’ abilities in critical thinking, conducting experiments, and properly analysing results. In the first author’s master thesis, a teaching concept for the beginning of natural science classes was developed and evaluated in a middle school in Vienna. The concept was evaluated and improved in three cycles using the design-based research method. Students should perform a simple experiment that leads them to measurement uncertainties. Sources for uncertainties and the trustworthiness of the experiment are discussed. Using simple methods of statistics such as the arithmetic mean, the experiment will be analysed. The tested class revealed positive outcomes. The students’ discussions during and after conducting the designed experiment provided valuable insights. Furthermore, linking the word trustworthiness to the technical term measurement uncertainties seems to be a promising way to introduce students to the concept of measurement uncertainties.