If you download often from Rapidshare, then you might be familiar with image verification code they put on download pages. These image codes are meant to stop computer programs downloading from their server.
Now if you think those image codes being easy, then check out following challenge. Of course this must be mock up.
If you really like to take above challege, check Wikipedia entry for Riemann hypothesis! 😉
Embarrassingly, I don’t remember who created above image as I saved it randomly long time back on my system. If you know who deserves credit, please let me know via comments. 🙂