And once you can reliably flip any object a half rotation, one/two/etc rotations, it transfers fairly easily to similar objects, and to the juggling pattern.

Going from not juggling to juggling knives can be done in perhaps weekends.

Then, of course, you can do all the fancy multi-object patterns, but I've yet to see a reaction to three spinning knives surpased by five+ tennis balls. The five+ tennis balls are neat while the three (even just two) spinning knives is always a visceral holy shit please stop you're going to hurt someone.