By that, I mean that her head is shaped like an egg. At least, that's how I've drawn her over the years:
Also, her birthday is near Easter, which reminds me of Easter eggs.
To celebrate her birthday (and her eggheadedness), I made this offbeat caricature on her noggin:
This video shows her Grade "A" reaction when she opened her crocheted gift:
Happy birthday, sis! May your shell be strong and your yolk be sunny!