The babies were shown a video of three women who were bilingual speakers of French and English—languages the babies didn't know. Each woman was shown in turn saying sentences in one of the languages. Eventually, the baby got used to this and got bored. Then the language changed. Monolingual babies took no notice, but bilingual babies started looking at the video again. Other studies have shown that monolingual babies make the distinction until they are 6 months old.
Hmm. I wonder if I noticed that slight change between two similar languages when I was growing up. My mom speaks Punjabi but my dad speaks Urdu.