During Apollo 10 — in which a crewed spacecraft orbited the moon as a dress rehearsal for the first lunar landing — astronauts encountered a deep mystery.

"Oh — who did it?" commander Tom Stafford suddenly asked, six days into the mission, as the crew discussed preparations for leaving the moon's orbit.

"Who did what?" inquired command module pilot John Young.

"Where did that come from?" interjected lunar module pilot Eugene Cernan.

A moment later, for listeners at ground control, the mystery was resolved.

"Give me a napkin quick," commanded Stafford. "There's a turd floating through the air."

It sounds apocryphal, but it's the truth: in an era when spacecraft waste management consisted of "a plastic bag which was taped to the buttocks to capture feces," turds could escape from time to time and float about the spacecraft.

The Apollo 10 astronauts proceeded to debate the provenance of the turd in question, with no one owning up to producing it:

And just nine minutes later, Cernan was flabbergasted to be interrupted yet again by another floating piece of feces:

The discovery was followed by another round of denial. To this day, the great space turd mystery of 1969 has never been solved.

Read more about the weirdest things we've left on the moon, or discover 9 surprising facts about feces you may not know.