What are some of people's favorite favorite riddles or "thinking puzzles" that they have used in the past? In a few weeks my party will be making their way through a series of trials to make it through to a sphinx's stronghold to petition it for help, and most of the trials involve being presented with trivia, riddles, or puzzles to solve to progress to the next trial. If all of the allotted guesses are used up, there are sentinels that come to attack the party to drive them out, but if they defeat the sentinels they may continue to progress, so there is a way to get past if they are absolutely stuck (proving themselves physically instead of mentally).

I have a list of some already, but all of my players are both GMs as well and engineering students, so they already know the answers of a lot of the more common dungeon puzzles (or could work through them pretty easily).