What was the joke that Robin Williams told by Waler Cronkite?

There seems to be a misunderstanding. There is no record of Walter Cronkite ever telling a joke by Robin Williams.

* Walter Cronkite was a renowned American broadcast journalist, known for his role as anchorman for the CBS Evening News.

* Robin Williams was a comedian and actor famous for his improvisational comedy and stand-up routines.

While both were prominent figures in their fields, there is no known instance of Cronkite sharing one of Williams' jokes. It's possible you're thinking of a different story or may have misremembered the details.

If you have more information about this specific joke, please provide it so I can help you find the answer.