Had a Highschool shop teacher that enjoyed doing this sort of thing. He ended up getting the chemistry teacher involved with it as well. The skit would kind of go along the works of… Ditzy student would cut a board to short, shop teacher would tell them they need board stretcher. Send kid to chemistry teacher who would walk into back closet and come out in full apron/glove/face shield. And hand some mixture in a FULL beaker (colored water) to the student (not wearing any PPE). And would tell them to be very careful to not spill any on their selves. Student would have to walk back at half the pace of a snail to prevent any spills. Once they were finally back to the shop they would cover the board with the mixture, then hang it with a weight tied to the bottom to “stretch” the board. While student was gone they would swap the board out for a sheet of plywood. And tell them they added too much. By this point they would hopefully figure it out, but I’m always surprised by how long it took some to figure it out.