Cross thread Exception

Question

I don't understand why I get a cross threading exception on this. Can someone plz explain?

Thread1 = new Thread(()=>UpdateLabel(ref label1));

Thread1.Start();

void UpdateLabel(ref Label _label1)

{

lock(_label1)

{

_label1.Text = "Hello World";

}

}

I thought that would make it a resource shared between the GUI thread and Thread1.