Can you guess what it is?
It's not been the best week for The Queen - she's fallen ill with a cold, spent the night in hospital as a precaution and had to cancel both a two-day trip to Ireland and her appearance at COP26 as a result.
And now, in more Royal news, it's thought that she's facing "heartbreak" after pain has forced her to give up one of her favourite hobbies.
It's been claimed that the past time had begun to cause her "discomfort", prompting her to stop the hobby altogether.
No, we're not talking about walking her beloved corgis, but rather, horse riding.
It's thought the 95-year-old monarch was forced to give up horse riding around two months ago.
On her summer holiday in Scotland, while staying at Balmoral, its thought she stopped riding as a result of "discomfort" but is thought to be "extremely disappointed".
A Balmoral source told The Sun: “She was in quite a bit of discomfort."
"She adores riding and it has been part of her ritual for most of her life."
"She has been extremely disappointed not to go riding since the beginning of September.”
It's thought that she's keen to get back on the saddle, but when hasn't yet been confirmed.
She's well known for her love of horse riding - she normally rides Fell ponies around the grounds of Windsor Castle and regularly appears at Ascot Races, too.
Not just that, but she personally breeds thoroughbred racehorses, too.
She's currently focusing on 'lighter duties' after her hospital visit last week - and while Buckingham Palace officials have confirmed the Queen is in good health and spirits, one Royal family member is thought to be in "panic mode" over her health concerns.
A Buckingham Palace spokesperson said: "Following advice to rest, the Queen has been undertaking light duties at Windsor Castle."
"Her Majesty has regretfully decided that she will no longer travel to Glasgow to attend the evening reception of Cop26 on Monday, 1st November."
"Her Majesty is disappointed not to attend the reception but will deliver an address to the assembled delegates via a recorded video message."
Hoping the Queen gets well soon.
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