Imagine Aliens need a Substance (like Salt, etc...) so Bad. -Mike666
"Limestone," said the Human with a raised eyebrow. "You guys need calcium carbonate? For reals?"
"The calcium carbonate is an essential mineral for us, yes. For reals."
The Human shrugged, "Well, we need sodium chloride in special amounts, so yeah. I'll try to see if we have the pure stuff for you." There was some significant rummaging about in the storage boxes. "Yeah. I usually have these for baby carvers, but I got you some of the more manufactured stuff. How much by weight are you looking for?"
"Oh," said the customer. "My crew and I will need a total of twenty Siwus for the entire journey plus contingency."