states of matter
I guess they are getting that there is varying level of motion. Solids are like students sitting a class room, liquids are like students going outside to play. Gas are like students rapidly moving around the playground. A thorough discussion is in the linked paper.
METAMIA is a free database of analogy and metaphor. Anyone can contribute or search. The subject matter can be anything. Science is popular, but poetry is encouraged. The goal is to integrate our fluid muses with the stark literalism of a relational database. Metamia is like a girdle for your muses, a cognitive girdle.