ping(Pong_PID) ->
    Pong_PID ! {ping, self()},
    receive
        pong ->
       io:format("Ping received pong~n", [])
    end.

pong() ->
    receive
        {ping, Ping_PID} ->
            io:format("Pong received ping~n", []),
            Ping_PID ! pong
    end.

start() ->
    Pong_PID = spawn(ping_pong, pong, []),
    spawn(ping_pong, ping, [Pong_PID]).