Back when I was in university, I bought several feet of black corduroy to use as a blackout sheet for my dorm window during exams, so that I could sleep in later when I had the opportunity.
So one night I put it up (we had aluminum double sash windows, so that the upper pane could be moved up and down) by having the sheet folded over the top of the upper pane, which I then closed as tightly as possible. Over that I closed my curtains.
Overnight, the upper sash had dropped a few millimeters and the curtains had opened about the same amount. When I woke up the next morning, on the wall facing my window was a very sharp, inverted image of the outside world; I had inadvertently turned my dorm room into a camera obscura. It was really cool at the time.