I want to Work with Multiple projects In One Solution Explorer

Question

Please can anyone help me with how to work with two or more projects in the same solution explorer.  That is I have project A and Project B with each having  .CS files.My problem is to be in project A and call the classes in Project B and vice-versa.  Please demonstrate with codes or a sample project because am new to C#.Net  or send them to my email :[email protected]