How is this done?

[christyf5]

Nope its not a trick. Because the "vertical" tank is sealed and not letting any air in, the water in that tank is "trapped" and has nowhere to go due to its submersion under the water of the horizontal tank. I can't remember what scientific principle this is called. Try it with a full pop bottle, immerse the open end in a sink full of water. The water in the pop bottle won't go anywhere unless you pull it above the level of the sink water so air can get in to the bottle.

I'm trying to think of practical uses and all I can come up with is my cats "auto water bowl". Basically the same principle, upended 2L water bottle in a dish of water, the water dish is always full until the 2L is empty.