Why is this code not working?

[Ivanno]

I have been sitting in front of my PC for 5 hours straight now trying to solve this, but it just comes to the question and if i answer yes it breaks the loop as wanted, but if I type in no it crashes. Any help is appreciated, thank you, -Ivan

def clss():
    import os
    os.system('cls' if os.name == 'nt' else 'clear')
def pause():
    raw_input("Press Enter to continue")
print ("No Life Ent.")
pause()
clss()
print ("The No Life inc. Quiz")
pause()
clss()
print ("Begin game?")
a1 = raw_input("Type in yes or no: ")
while a1 !="yes":
    if a1 == yes:
        print ("Ok, let us begin then")
        break
    else:
        print("Type in yes.")
        continue
pause()
print("THANK THE LORD THIS WORKS!")
pause()

I've got Python 2.7.13 if that is of any help.