Selection made to one ddl to populate another ddl's items, no database connection required

Still a rookie, here. ASP.Net WebForm, using C# with Visual Studio.Net. Two drop-down lists (ex: ddlContinent and ddlCountry) on same WebForm. What do I put in selectedIndexChanged event of either ddlContinent or ddlCountry so that when item selected in ddlContinent populate items list of ddlCountries without having to use a button click event? I do not need a database connection. Can anyone help me? Reg