Returning to work may prompt us to wallow in self-pity, but taking a look at a thread on social networking site Reddit may be just the ticket to put it all in perspective today.

Over 2500 people have responded to the question "What's the saddest fact you know?" with a plethora of users' most depressing snippets of knowledge - including lonely whales and depressed 9/11 rescue dogs.

RamsesThePigeon shared how the Mars Rover celebrates its own birthday by singing Happy Birthday to itself.

In August 2013, it was confirmed that the Curiosity Rover was programmed to sing the song on the appropriate date while alone on the Red Planet.

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The same user also shared a sad Yuletide fact: "In 2012, one of the most-requested Christmas gifts (by children in the United Kingdom) was 'A father'."

A few philosophical notions were offered with Fred1840 writing: "There will be a moment in history when you will be thought of for the last time ever."

While NotSoGreatGonazo shared: "If you're really lucky, you meet the right person, and then you live together until one of you dies. And that is considered to be the best case scenario."

Many cited heartbreaking facts about the animal kingdom.

Mixtapeshuffle wrote: "Whales that sing in the wrong key get lost and are alone in the ocean."

A documentary which began in 2015 focused on a "52-hertz whale" who calls at an unusual frequency and has been nicknamed "the world's loneliest whale".

Unkyrona said: "Cows have best friends. So when the friend is gone the cow becomes depressed."

A 2011 study at Northampton University showed that the mammals become distressed, with increased heart rates and cortisol levels, when separated from their "friends".

Some referred to childhood facts such as Yossarian who lamented being picked up by a parent.

Another user decided the saddest fact they'd ever heard was about a childhood voice actor killed by her father.

User SoundsBearier shared a post about Judith Barsi, who voiced Ducky in The Land Before Time, and was murdered by her father before the movie was released.

"The details of the story are more heartbreaking than the synopsis I've just given you. :-( Read about it, then listen to If We Hold On Together and just try to keep yourself from falling to pieces."

The child star was 10 when she was shot in the head while she slept by her alcoholic father.

- nzherald.co.nz