It's simple and fascinating! Once you created the terrarium and closed the jar a mini water cycle establishes - you don't need to add any water. During the day the plants convert O2 to CO2 and during the night CO2 to O2. The only thing they need from outside is a bit of sunlight.
Don't place the jar in direct sunlight, so the water cycle stays intact. Instead place it in a shady spot.
Oh, and don't place living snails in them - that's only a shell in the video :D
Much fun creating your own tiny world!
