F(x)'s Krystal and Sully posted a playful photo of them together.

Today via Instagram, Krystal put up a photo of her and Sully with a one -liner, "forever young."

In the photo are the two of them with their eyes and mouths wide open, feigning a shocking surprise. Sully's boyish denim jacket and orange snapback hat has cracked a few laughs from the viewers, some of whom commented "this made my day" "do not grow old and boring, girls!"

If growing old means to stop laughing and being silly, Krystal and Sully are showing us that it is good to be young, and better yet, forever young.