I saw this at Filipino Programmer’s blog. Notpron claims to be the hardest riddle available on the internet. Is it true? I don’t know. But the riddles there are indeed hard, I can’t even pass level 3. Hehehe.
Googling notpron solutions will give you more than a thousand links on how to solve it. But I don’t want to give in yet. I will try solving it during my spare time. At least, I pass level 1. Hehehe.
If you have nothing to do, visit the site and exercise your brain cells. It has nothing to do with porn, so, don’t worry.