In this activity, learners discover that there is space between molecules even in a cup "full" of water. They first fill a cup with marbles, and then add sand to fill the gaps between the marbles. This serves as a model when they add sand and salt to a full cup of water, discovering that more salt will fit into the tiny spaces between molecules. Provides an opportunity to discuss the use of models in science, and the benefits and limits of marbles as a model for molecules. Extensions include trying different substances, and comparing densities of saltwater and plain water. Part of the "No Hassle Messy Science with a Wow" activity guide by OMSI, where all activities use only household materials.