The smartphone is placed in a (tightly inflated) plastic bag. Through the pressure sensor, you can determine the changes in pressure when differen weights are placed on the bag.
This experiment was shown to us by Holger Jessen-Thiesen (Twitter), who also provided a worksheet in German, which has been translated by Solmaz Khodaeifaal (Simon Fraser University). The link at the top refers to a PDF version, but you can also use this docx file to adapt it to your needs.