The heartbreaking discovery in Marge Piercy's "Barbie Doll" is that the girl in the poem has "a great big nose and fat legs." This might seem a trivial matter, since she is healthy and intelligent, but, over time, it affects the way in which people treat her to such an extreme degree that she no longer wants to live and cuts off her nose and legs.

The speaker conveys the complexity and impact of the terrible discovery through literary elements including simile and irony. In the third stanza, she says that the girl's good nature wore out "like a fan belt." This provides a vivid mental image of something essential to her survival suddenly snapping after years of wear.

Irony is a particular feature throughout the poem. The heartbreaking discovery is presented in the first stanza as part of "the magic of puberty." In the final stanza, the dead girl is described as enjoying "a happy ending," since she is beautified in her coffin to look like the Barbie doll she never resembled when she was alive.