Why the same sound is a b to English ears and a p to Spanish ears

Say "bat" and "pat". The only physical difference is a tiny delay between your lips opening and your vocal cords buzzing, about 12 milliseconds for an English b and about 60 for a p.

Your brain turns that smooth dial into two boxes, and where it draws the line depends on your language. Slide the delay yourself and see how the same sound flips between "b" and "p" for English and Spanish listeners, and why babies hear more than adults.