She just never learned how to sit on the couch.
It's not her fault. The youngest daughter in a family of professional stock photo models, growing up surrounded by flashbulbs and fashion doesn't afford one many opportunities to sit on the couch like a normal person. Decorated by the most accomplished avant-garde designers to maximize feng shui, ergonomic sitting spaces were the order of the day. The couch, as you and I know it, did not exist in her world.
The lifestyle was stifling, so she broke free. She went to college and earned a doctorate in physics. She bought a PlayStation 2, got into poker and half-heartedly eating pizza.
She purchased a couch.
Come on, give her a little credit. At least she's trying.
She's even managed to recruit her identical sisters to her rebellious cause. While they share her inability to sit properly, they've mastered playing poker and puffing away at an unlit hookah.
And would you look at that! With her sisters pinning her legs behind their backs, she's actually managed a sitting position! Let the pride of that triumphant shout wash over you. Imagine the scene playing out in slow motion with the theme song from Chariots of Fire playing in the background. One can almost make out a cheering crowd in the blank space over their heads.
Now all she has to do is maintain that pose when they leave, and she'll be a couch novice no longer!