How+形容词/副词(+主语+动词)!如:How fast it flies!How beautiful it is!其中主语+动词可省略.
所以:
How interesting the joke is!
What+a/an+形容词+单数名词+it is! What +形容词+复数名词+they are! What +形容词+不可数名词+it is! 其中it is/they are也可省略.如:What a tall building! What an interesting book! What nice weather! What kind girls!
所以:
What an interesting joke!
同一楼上，用funny较好些~