The U2 Riddle, one of my favorites, has been posted on this site for a few years now. It’s one of the entries that has drawn lots of traffic, and is in fact the #1 result on a Google search for “u2 riddle”.
A few weeks ago, my cousin Erin and I were driving back from Canada from our cousin’s wedding. While stuck in traffic at the border for nearly three hours, I threw the U2 Riddle at her. It’s a beautiful riddle, and at one point was used by Microsoft to screen prospective employees. Based on Microsoft’s demands to find the solution in five minutes or less, it looks like Erin’s got what it takes to be a Windows engineer. She immediately found the key point to the solution (i.e. Adam and Larry cross together) in one try, channeling the critical thinking skills of our surgeon grandfather. The rest was just a matter of walking through the details.