What would MacGyver do if he needed a high voltage capacitor but only had some foil, tape, water, salt, a nail and a plastic jug? He would build a salt water Leyden jar, that’s what. The Leyden jar is a very simple capacitor. Invented in 1745, it has been integral to many scientific experiments. Check the wikipedia entry to learn more about the history.
This specific type is quite easy to make. It uses a salt water interior instead of foil on the inside and outside. That means you could slap one of these together in a few minutes to impress your friends and/or electrocute yourself accidentally. Please be careful as this is high voltage.