Hi, and welcome to JA!
The answer to this riddle is "No".
Simply, it is based on the last line "Can you guess the riddle?" You can't guess it, so "No". That's why kindergarteners got it correct -- because they were honest! (versions of the riddle mention how most Harvard grads get it wrong, but kindergarteners get it right)
There are many sites on the web that are debating this riddle, and here is one: http://channel9.msdn.com/ShowPost.aspx?PageIndex=19&PostID=184297
I hope this helps,