Problem with tkinter button

Hi I am new here,
I am having some issue with a simple code:
import tkinter as tk
r = tk.Tk()

def Opencalc():
import os
os.system(’“calc.exe”’)
r.title(‘Open calc’)
button = tk.Button(r, text=‘Open’, width=25, command=Opencalc, activebackground=“red”)
button.pack()

r.mainloop()

I am just learning python, when i click on the button, it open calc, but the button remain pressed why?
I want the button to be released after the calc is opened