Don't get started on these things while at work, otherwise you'll kill your entire day and get all pissed off. In both of these games, the idea is the guess the URL of the next page in the series.
Exhibit A is a puzzle where the answer is written in a clear form, it's just disguised in some way. The first little bit is fairly easy, but it gets pretty tough pretty fast.
Exhibit B should only be tackled if you have a week to kill -- at the bare minimum. Over 1.5 million have tried it, only 173 people have completed it. It involves digging through hex code, images, opening up sound files, as well as a pile of other pain in the butt stuff, sometimes to get passwords to move onto the correct page. It has 81 levels, and a huge forum if you're having trouble with them. Just don't ask for hints. I've gotten through eight of the levels, and I was pretty proud of myself, but even that took a long time.
I'm just not going to get started on any of these things, otherwise I'll be at it for quite a while.