How a young toddler adapts new words/languages quickly

Human child
Age-3.5 years old
Gender- Female

I have been closely observing a toddler since the past 6 months and her ability to comprehend languages is astonishing. By just sitting in front of the tv for 2 hours she has learned many new words that are usually taught at the age of 7-8 years, the most wonderful part of this observation is that the kid is able to understand where she can use the word, she reads the situation and uses the developed vocabulary accordingly. The incident I want to share is that : Once we were discussing about how a particular thing looked like, the kid was present there too and was listening while playing around. When I asked her casually about her opinion , she spoke " it’s vague." It’s amazes me that she really understood the situation and spoke accordingly, similarly each time her parent leave for office she is sure to tell the, " don’t worry, i wont open the door to strangers."