Workplace hijinks.

Few days ago at work, it was a slow day, so one of the service guys was cleaning up his area, including vacuuming out the dirt and dust that had accumulated in the spare parts storage shelves. The other service guy digs out a motor speed control, puts some plugs on it real quick, and during a time when the guy had the vacuum cleaner turned off, inserts it into the power cord feeding the vacuum cleaner.
So every so often, as the guy is vaccuuming dust off the shelves, the speed of the vacuum cleaner would unpredictably slow down and speed up. I was hearing the varying noise from around the corner while I was working, and it was puzzling me too. Messing with your coworker's mind, great fun. I was amused when he showed me the speed controller the next day.