Yiting Cheng might have the coolest thesis project I've seen in awhile; for her master degree in design she created eight objects that store secrets. In her words:
This project is about concealing valuables, secrets, bad habits and personal information in our workplaces. Here, hidden spaces/messages were created within 8 general objects such as wood boards, lamps and disposable coffee cups. How? By utilizing stereotypes and visual camouflage. We make judgments based mainly on our experiences and what we see. This dependency on visual information can create large blind spots. Thus, usual stereotypes of how we perceive solid, transparency and lighting are employed in this project to play with notions of "solid and void" and "true and false."