SALT

“Salt,” said the Doctor, “is prized everywhere but here. On Earth, for example, salt was once considered so valuable that soldiers were paid in it. Wealth and social status were measured by whether one was seated at table above or below the salt. It was also used as a weapon to render enemy soil barren.”

“Why is it not valued here, then?” Nyssa was baffled.

“Because the dominant life form here is bacterial. Salt is virtual poison to these folk.”

“Bacterial!” Now Nyssa was worried. “Won’t they infect us?”

“Oh no,” laughed the Doctor. “We are too salty for them!”