Water

Cybertron gave up water eons of your Earth years ago.

Water is a chemical so rare even its existence is suspect on Cybertron, or at least so claims Ratbat. It cures Scraplets. The Cure!

(Note: Ratbat is likely confused; as several humans later managed to exist without environmental protection gear on Cybertron without drying up like old corn. Also, Ironhide mentioned his 'water gun' right in the very first comics.)

From Teletraan I: The Transformers Wiki, a Wikia wiki.