Cameron Diaz is in phenomenal shape, with her athletics and workouts giving her a tight movie star physique. In the movie What to Expect When You’re Expecting, she wears a prosthetic for a pregnant tummy and swollen breasts as an expectant mother. In real life she learned why some women’s breasts sag after they give birth, and she shared it with the press in a good spirited anecdote.

“Does anybody know why boobies sag?” Diaz polled. “I was in Africa and the Maasai women after they give birth, they go right back to work a couple hours later. They just kind of strap the baby on their back and keep going. Well, when you have a baby on your back for a year and a half, when you’re nursing, what you have to do, you can’t stop working. What you do is you throw your breast over your shoulder and the baby is strapped to your back so it suckles.”

Hearing this story, Diaz’s costar Anna Kendrick covered her eyes and started to gag. Unphased, Diaz gave further examples of on the job breastfeeding.

“Or, if you have the baby on your back and you spin it underneath your arm so the baby can take it behind and suckle while you still keep working,” Diaz continued. “So there’s an actual function for saggy titties. It’s because you have to feed your child. Isn’t that funny why we have boobs? We have them so that we can feed our children. I know there’s a belief that you’re not supposed to feed your children with your breast milk but that’s what it’s for. It’s functional. It’s not just a trend. It’s functional.”

Finally, Diaz mimicked a demonstration with an invisible baby on her back. “Over the shoulder and they literally go in and keep working and the baby’s just [makes suckling noise.] I just wanted you guys to know.”

What to Expect When You’re Expecting opens May 18.