require 'tk'

TkButton.new(nil, 'text'=>'new window', 'command'=>proc{ new_window }).pack

def new_window
  w = TkToplevel.new
  TkLabel.new(w, 'text'=>'new window', 'padx'=>10, 'pady'=>10).pack
  TkButton.new(w, 'text'=>'dismiss', 'command'=>proc{ w.destroy }).pack
end

Tk.mainloop