Imagine you're alone in space, your first time and completey alone.
When unexpectedly you hear a knocking sound.
That's what happened to Yang Liwei, China's first man in space, on his maiden flight in 2003.
In a recent interview, he has now recalled hearing someone knocking the body of the spaceship just as knocking an iron bucket with a wooden hammer.
It neither came from outside nor inside the spaceship.
Naturally, he got a bit nervous and had a peek out the porthole but failed to spot any explanation for the eerie knock.
He's not been able to figure out what it was, neither up in space nor after returning to Earth.
He has even tried, but failed, to recreate the sound so that experts could help him identify it.
Unsurprisingly, the story about unexplained mystery sounds in space has gotten quite a bit of attention.
What, or who, was knocking on Mr Yang's spacecraft as he was all alone miles from the safety of the Earth?
Given that there is no medium for sound to travel, space would be expected to be silent.
The travelling of sound travels requires a medium, be it air particles or water molecules or metal, solid atoms, Prof Goh Cher Hiang, an expert in space engineering at the National University of Singapore, mentioned.
Simple examples for this would be thunder, sound travelling through air, underwater sonar, or a solid musical instrument solid.
If it is knocking, there could be something physical hitting the spacecraft carrying the astronaut, Hiang says, but stresses that any such suggestion is purely speculative.
His colleague Wee-Seng Soh brings a different explanation to the table, suggesting it could have been a result of expansion or contraction of the spaceship, especially since the temperature of the spaceship's exterior could change considerably within the orbit.
Mr Yang's days as a spaceman are long in the past but according to Chinese media, the sound has also been heard by subsequent Chinese astronauts on missions 2005 and 2008.
In fact, the veteran told his successors about it so they wouldn't be caught off guard and worry about it like he did.
So even though unexplained, he now puts it down as a normal phenomenon.