According to the film's director it's because she is cooking eggs. Seriously.
“I wanted to show the way she dreams of water, uses water to boil her eggs, and then goes and gets in the water, and masturbates, shines her shoes, and goes to work. A perfectly acceptable routine by any standards,” he said, chuckling to himself. Elisa’s masturbation is part of her daily routine, becoming almost rote in the flow of the movie. Del Toro wanted to show female sexuality as something natural, which he recognized is so rarely done onscreen.
