Your three-year-old will say a word you never taught them. Usually inside the first month.
Your kid is on the kitchen floor. Pressing something. Then says "stethoscope." Or "pineapple." Not from you. Not from Ms. Rachel. Off a cardboard book that costs less than a single speech session. Once you hear it, the iPad stops coming out.
The question every parent asks next is where that word came from, because it did not come from you.