Make a panel visible on Form1 by clicking a button on Form2

Another way to do it from Form2 (no changes needed to Form1):

private void btnShowPanel_Click(object sender, EventArgs e)
{
   Form1 f1 = (Form1)Application.OpenForms["Form1"];  
   Panel p = (Panel)f1.Controls["panel1"];
   p.Visible = true;
}