A phoneme is the smallest unit of sound in a language. Babies are born with an ability to express themselves with all the phonemes used in any language. However, nowhere is the "use it or lose it" concept more applicable than here. Children learn to use certain phenomes that are applicable in their local language. The other phonemes are lost.

However, this loss is total. It is not like swimming or riding a bicycle. It does not return at a later stage. It does not return. The child who learns a second language at a later date has to actually learn how to pronounce the relevant phonemes. All too often he never quite gets it right.