Someone in a private message asked me how lightning in a bottle works so I put together a demonstration and animated my theory of how it works. It's also a neat demonstration for use with a Van de Graaff generator.
Doing it is very simple. Fill a plastic bottle with water and put it on top of a Van de Graaff generator. Then step away and turn the Van de Graaff on. Smaller Van de Graaffs will need to use smaller bottles. At first the sparks will be short, but they'll gradually get longer.
The video below demonstrates the lightning in a bottle in action. It also shows the setup in case you want to try it yourself. And of course it illustrates how it works.
Warning: The bottle becomes very charged on the outside and on the inside. To discharge it, I turn off the Van de Graff and discharge the dome as normal with a grounded stick. Discharging the plastic of the bottle is harder though since plastic is an insulator and touching it will a grounded stick will discharge only the spot you touch, the charge won't move around on the bottle. So I pick it up with a rubber glove with one hand and run water over it in a sink to discharge it well. I also open it up and pour out the water while pouring fresh water in.