"Mystery boxes are a great tool to practice sci­­­entific reasoning skills and introduce students to the power of scientific models. Students develop hypotheses about the internal structure of a mystery box and come up with ideas on how to test their hypotheses. They also learn about the difference between observation and inference. Below, we present ideas for activities using 3D-printable mystery boxes. If you don’t have access to a 3D printer, you can also build a similar setup using cardboard or build a mystery tube with two ropes. We know it’s tempting but NEVER open the mystery box, that’s not how science works. If you cannot resist the temptation, glue the box together (that's what we did). The 3D-printable setup We designed a set of 11 different 3D-printable mystery boxes. The setup includes base: a circular base with different option of internal structures, for example, a square, triangle or a line steel ball: 5 mm diameter, is placed inside the structure in the base, needs to be purchased separately (look for a shop selling ball bearings) lid: to close the mystery box, we recommend glueing the lid onto the base small neodymium rod magnet (such as S-05-14-N): for more accurate observations In some cases, it makes sense, to label the mystery boxes, e.g., with a specific number, before you glue them together. In this way, you always know, which structure is inside. However, we usually try to avoid labels unless necessary because that’s not how nature works. Suggestions for educators There are different ways to use mystery boxes in your classroom. Depending on the age and level of your students, you can adapt the activity to different difficulty levels. Easy version: Prepare mystery boxes with the easiest internal structures: triangle, square, line, or an empty base. Ideally, every pair of students get one mystery box. Let the students study their mystery box and instruct them, to come up with a model of its internal structure. Here, students observe the sound the steel ball makes when moving the box. Let students draw their model on paper. Now 2 pairs of students with different drawings work together. The students explain their models. Then, they come up with ideas to test each other’s model. These ideas should be predictions about the behaviour of the boxes in a certain experiment, based on their model. For example: “If there is a square inside, I should hear 4 clicking sounds when I turn the mystery box one full turn.” Students test their predictions and note down their observations. If needed, students improve their models if the predictions did not match their observations and draw their improved model on paper. Next, you can introduce the principle of probes and more accurate measurement with the help of small rod magnets. Every group of students gets a small rod magnet, that will attract the steel ball if it is close enough. With the help of the magnet, students can again test their models of the internal structure: they will feel if the steel ball cannot be pulled in a certain part of the base because there is a hidden wall. Difficult version: Instead of the easiest internal structures, used the more difficult ones. Game version: We also use mystery boxes in our S’Cool LAB escape game. Here, we present 4 symbols representing the internal structure of the boxes in a certain order, e.g., ▲OI◾. The mystery boxes are labelled with numbers, e.g. ▲=1, ◾=3, O=7, I=8. Students have to determine the internal structure to place these 4 number in the right order (e.g., 1783) to open a combination lock and find the treasure or the next clue."