The wardrobe secret was first revealed to many via a tweet from Vulture writer Kathryn VanArendonk, who wrote: “I think it’s important everyone know that at the virtual premiere event [for The Crown] this week, helena bonham carter cheerfully announced that marion bailey who plays the queen mum wears a body suit where her breasts are filled with birdseed.” [sic]

This is also something that has been confirmed by Bailey herself, who told the Daily Mail: “I have this marvelous bra which is padded with lentils and birdseed, I have no idea why but I daren’t get too close to any bird tables in it!”

Padding bras with birdseed to replicate an older woman’s breasts may seem strange, but it is a costumier’s secret that dates back at least as far as the Victorian stage.

Other recent TV actors who have found themselves transforming into a character thanks to a birdseed bra include Sarah Paulson. In a 2014 Comic-Con panel, Paulson described playing an 80-year-old woman in the finale of American Horror Story: Coven.

She said: “It’s really hard to play an 80-year-old woman…It was the scariest thing I’d ever done, and I remember Ryan Murphy being very concerned about the old lady make up what it was going to look like… I looked exactly like my grandmother.. I had these birdseed boobs they gave me.

“They’re just sacks of birdseed that feel like big old tits that have fallen down. It was really amazing. I was like, This is awesome. This is coming for me if I’m lucky. One day I’ll have a sack of birdseed.”

In May 2015, meanwhile, a theatrical costumier wrote of what are known in the business as “birdseed boobs” on their blog. “When we wanted to give a female character large and/or sagging breasts, without a complete bodysuit, we used to make a pocket in each large bra cup and fill them—but not tightly—with flaxseed, which simulates the weight and movement of breasts.”

They added, “One thing I learned from a friend: never, never put birdseed boobs into storage without removing all the birdseed . . . . Unless you want mice nesting in your costume storage area.”

Bailey is not the only person who has shared secrets about how they turn into their Crown character. Gillian Anderson recently shared a series of Instagram shots of her turning into British Prime Minister.

Last year, meanwhile, Netflix shared a video that revealed the make up process that turns Olivia Colman into the Queen.

The Crown Seasons 1 to 4 are streaming now on Netflix.