Why are Jews funny?

I don't understand why saying the word "Jew" is funny.  In South Park, the word Jew is used all the time, without really condescending Jews (although it does that as well).  In Mr and Mrs Smith, Mrs Smith states "I'm Jewish" and the audience laughs.  Other instances of random Jew references include SNL, Simpsons, etc.  Then you have Jews who make Jew references, including Mel Brooks and Woody Allen. 

What is so funny about mentioning Jews? 

I don't see how it's not racism.  Doesn't poking fun at Jews imply that they're inferior to us, and have strange customs that prove their religion is just silly?