Many children in many communities in the world grow up in communities in which the language situation is increasingly fluid, and where the acquisition of more than one language is the norm.
People move between languages, depending on who they are talking to, and what they are talking about.
In language contact situations, new languages may emerge, sometimes categorisable as creoles, sometimes as mixed languages. These languages enter the repertoire of languages that speakers move between.
Children growing up in such communities hear their sisters, brothers, parents and friends and relatives speak traditional languages as well as the emerging languages, and they hear people switch readily between these languages.
How children in these multilingual environments learn to talk, what languages they use, what languages they hear, and how they interact with people in such communities are what we are calling "contact acquisition".