Researchers have discovered that being bilingual changes how the nervous system responds to sound.
The study in the Proceedings of the National Academy Of Science compared bilingual teens and those who only spoke English. Those who could speak both English and Spanish were better able at converting speech sounds.
What seems to happen is that the added experience with language "fine-tunes" the nervous system that governs hearing. It also enhances attention and working memory, making a second language something to talk about.