Before creating a situation with a real "interaction", our tasks recommend assembling an object and comparing the mass of its parts to its total mass.
Students measured the mass of their creation. Then, they broke apart their creation into 2 or 3 parts. They measured the mass of each part and added to see if they totaled the the mass of the whole creation.
Generally speaking, all totals were correct. There were a few differences, but we agreed that our scales are a little sensitive, so that probably accounted for it.
The last step was to display their data. There were many great ones, but I forgot to tell them to put their names on the back, so I can't share them here! Here are some examples: