The answer to that riddle is: when you cry over spilt milk.
You can also get the answer by solving this math activity.
More often than not, well written riddles make good use of four linguistic devices: paradox, pun, poetic description and red herring.
A pun--the deliberate jumbling of the multiple meanings of a given word--is often necessary to create a paradox, so also is a red herring often needed to create a pun. Find a series of riddles that emphasize the use of pun on Theislandenglishtutor.com.
Tip! The single most important thing to remember when attempting to solve hard riddles is that they are meant to trick you.
Check out more tips on how to solve riddles from Howtodothings.com.