The Great Disappearing Celery Mystery, also known as the Celery Theft Mystery or the Disappearing Celery Mystery, referred to the low level theft of celery from the greenhouses and gardens of the first human colonists on the planet Sphinx.
It was at first considered to be a hoax or a prank, although speculation among the colonists at the time was wildly varied. It was a young girl named Stephanie Harrington who solved the mystery when she made a photographic recording of a treecat raiding her parents greenhouse. This resulted in the discovery of the previously unknown treecat species and the fact that they were responsible for the celery theft. (HHA1.1: ABF)
It was not until treecats learned sign language in 1915 PD that the ultimate reason why they stole celery -- besides their liking its taste -- became known: the gene-engineered celery had fixed a trace element which was vital in maintaining treecats' empathic and telepathic abilities. (HH10)