A bag of coins. The vesicles often contain important molecules, eg neurotransmitters. When they are released by the cell they often will have a physiological affect outside of that cell. This process is somewhat like throwing a bag of coins into a busy street, the action would cause the behavior of people seeing that bag to change.
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.