It's amazing to think that in the first year of our life we are so sensitive to the world around us. One aspect of that early stage is universal adaptability. It's the astonishing ability babies [up to the age of 1 years old] have that makes them able to distinguish small variances in vocal tones. It's how we start learning a language. However, somewhere after our 1 year of life, the ability is lost once we start to focus on one specific language.
I myself am bilingual. My parents are from Thailand and they taught me Thai and English simultaneously. Unfortunately my English is stronger than my Thai so I don't know if I'll be able to teach my future children Thai. I played around with the idea that I could just play hours upon hours of Thai cartoons so they would just pick it up. That idea was smashed to pieces when I learned that you need face to face learning to really pick up a language.
So, I'll just have to make sure to speak Thai around my kids if I want them to pick it up.