To get into heaven you have to walk up 100 stairs but on each stair God asks you a joke and if you laugh you go to hell. 

So the brunette gets to the 56th stair and bursts out laughing and gets sent to hell. 

The red-head gets to the 97th stair and bursts out laughing and gets sent to hell. 

Then the blonde gets into heaven and bursts out laughing. God asked her "Why are you laughing?" the blonde replied "I just got the first one!"