atom hitting a moving piston
"an atom hitting a moving piston is like a ping-pong ball hitting a moving paddle." The link is whole book on using analogy and metaphor in teaching science.
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.