This week, the Internet lost one of its favorite animal celebrities — and our hearts are broken.

On Sunday, Nellie the sea otter — famous for a viral video of her repeatedly stacking cups — was euthanized after suffering an unspecified acute illness. According to Kiro7, representatives from the Point Defiance Zoo and Aquarium in Tacoma, Washington told the Associated Press that veterinarians are still waiting on test results in order to diagnose the cause and nature of her illness.

For those unfamiliar, in 2012, a video featuring a tiny otter and her stack of cups stole the Internet’s heart — and Nellie was its star. In the video, the otter is repeatedly handed three small cups to stack, and then given a treat each time she successfully completes the task. (While Nellie learned how to do this before coming to Point Defiance, her trainers continued doing it with her in order to keep her “physically active and mentally alert.”) At one point, her trainer gives her the cups out of order, and Nellie adorably bangs them together in frustration.

After the video came out, Nellie the otter quickly became the mascot for online rage; and GIFs of her side-eying her trainer circulated far and wide across the web.

Here she is enjoying a fishy treat between cups.