Gyroscopes. If you think you understand them then you almost certainly don't. Eric Laithwaite showed this thirty-odd years ago. Indoors. Where the consequences of inadvertently dropping it can be very exciting indeed.
He was a controversial character. It was argued hotly at the time whether his gyroscope demos (there are quite a few on Youtube) clarified or confused people's understanding of the subject. The fact is that gyros don't break any laws of physics. But predicting what they will do requires a grasp of dynamics that very few people indeed ever achieve.