We went the 3D route using magnetic marbles to represent the walls and lids of the cup (solids are made of molecules, too). And we used glass marbles to represent the water in the cup (which we know can evaporate and possible leave the cup system if given the opportunity)!
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We noticed a bunch of things from this...
1. The water that attempts to leave the cup system by evaporating can't leave. This means that it is trapped, and the mass of the cup can't change.
2. If any mass change occurs, it must have come from outside the cup!
3. The walls/lid of the cup seem to move a little when the water that's attempting to leave the cup system hit it. Interesting!