The Basics
Your Baby's Pee Reflexes and
Brain Development

Your baby must be physically developed with a sufficiently mature nervous system before you can start training him. Your newborn pees all day and night because his bladder is very small, even a small accumulation of pee can cause his body to expel it as a reflex action.
 
However, as your baby grows, his bladder capacity will increase, and his body will start to produce a hormone that prevents him from peeing at night. At the same time, as the brain develops, it sends out signals like "The bladder's full!", or "It's time to pee!" after the bladder tells it that pee is accumulating.
 
This is how a child starts to learn what the urge to pee means. It's also the first step to him becoming independent of diapers.
 
Nowadays, toilet training is no longer being described as "taking off diapers", but "coming off diapers" instead. The idea of a toddler coming off diapers naturally when his brain and body reach a certain stage of development has gained widespread acceptance.