Researchers at the University of British Columbia have shown that babies can tell when a person switches to a different language just by watching the speaker's face. Researchers showed infants videos of three bilingual speakers reciting(朗読する)sentences. After being trained to become comfortable with a speaker reciting a sentence in one language, babies aged 4 and 6 months spent more time looking at a speaker reciting a sentence in a different language - demonstrating that they could tell the difference. Infants will likely lose this ability before their 1st birthday if they're not continuously exposed to language switching.