What is wrong with my Python threading?

[Chuckk Hubbard]

#!/usr/bin/python

#why doesn't this run both threads simultaneously?
#Thanks for any help.
#Chuckk

import threading
import time

def printesc(thrd):
    for i in range(10):
        time.sleep(1)
        print thrd, i

def master():
    thd1 = threading.Thread(target=printesc, args=(1,))
    thd2 = threading.Thread(target=printesc, args=(2,))
    thd1.run()
    thd2.run()

master()