On Betty White's birthday, fans have flooded the internet with a gift to the late star's legacy by honoring the cause she cared about most: her furry friends.

The Golden Girl is continuing to give the world joy, even after her death.

On January 17, beloved comedienne and actor Betty White would have turned 100. She died last month on New Year's Eve, just two weeks shy of the milestone.

The passion closest to her heart? Animals.

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When asked in a late night TV interview what she liked to do in her free time, she told David Letterman that she liked to "play with animals, mostly."

Throughout her eight decades-long TV and film career, she served on the board of American Humane, fundraised and volunteered for numerous animal organizations, and created her own weekly TV show, The Pet Set.

Now, fans are giving back in what could be Betty's most impactful project for animal activism yet.

The aim of the #BettyWhiteChallenge is to donate $5 to an animal rescue organization in White’s name on her birthday, and the hashtag has taken off across social media.

It is unclear just how the movement took off, but it's quickly gained attention and been promoted by celebrity friends like Mark Hamill and George Takei.

Betty's birthday will also be marked with a film in movie theaters on Monday – a documentary planned for her big day that is now a moving tribute. It includes Betty's final message to her fans, filmed just 10 days before she died, along with behind the scenes footage and interviews with famous friends. Betty White: A Celebration will be shown in 1,500 theaters nationwide.

Her hometown in Illinois also held Betty White Day on Saturday.

Despite her many awards and lifetime achievements, it was her work with animals that Betty perhaps remained the most proud of.

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"I have been a pet lover all my life," the 99-year-old posted on Instagram earlier this year.

In a 2011 interview, Betty revealed that she liked animals more than people, and that it was a secret to keeping her young at heart.

"I learned it from my animal friends: kindness and consideration for somebody besides yourself," she said. "And I think that keeps you feeling young, I really do."