Studies have shown the potential of explainer videos. An alternative is a written explanation, as found in science textbooks. Prior research suggests that instructional explanations sometimes lead to the belief that a topic has been fully understood, even though that is, not the case. This 'illusion of understanding' may be affected by the medium of the explanation. Also, prior studies comparing the achievement from explainer videos and written explanations come to ambiguous results. In the present experimental study (video group: n[subscript V] = 78, written explanation group n[subscript W] = 72), we compared the effects of an explainer video introducing the concept of force to a written explanation containing the script of the video, as though it is a page from a textbook. Both groups achieved comparable degrees of declarative knowledge, however, the written explanation video group had a significantly higher belief of understanding (partial [eta][superscript 2] = 0.043) that did not correspond with their actual learning progress. Consequences of this may include lower cognitive activation and less motivation in science classrooms if learning environments exclude further learning tasks that allow for a more realistic picture of understanding. That might suggest that it is sometimes potentially harmful to leave physics learners to their own devices with instructional material.