A few sessions ago, I gave my players a puzzle that involved converting a decimal number to binary to figure out the correct sequence to open a door. Since my group consists primarily of computer science students, I feel that the puzzle was enhanced by the fact that it was tailored specifically to their field of interest.
However, there is also a mechanical engineering student and a psychology student in our party, and I'd like to design some puzzles related to those fields as well, to allow them to take the limelight and feel like they're really contributing to the party with something only they can do.
Specific puzzles would be nice, but I would also like to know what kind of problem would be relevant to someone studying this field. For example, converting between binary and decimal is something every computer scientist knows how to do, and it often comes up in the course of a day. What would be some analogous, fairly simple puzzles for a mechanical engineer or psychologist?