parts per trillion
"parts per trillion half a teaspoon of salt in 5 swimming pools of water"
"One industry expert I consulted explained it to me like this: if parts per million is like finding the proverbial needle in a haystack, parts per trillion is like finding a microscopic mildew spore on a piece of chaff in that haystack."
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.