Have ever forgotten to water your plant and because of that it died? This will prevent such things from happening!
Step 1: History
Firstly, I will tell you about the history behind it. In the XVII century rich people wanted to have exotic plants. But they was a problem: they often died during the transport. But then, there was a guy, Nathaniel Ward. He had nothing to do wiht plants, he liked butterflies. He put baby butterflies in glass containers with the bottom filled with dirt. One of them was forgotten for few months. And in that time, a fern sprouted in the container! The water in dirt evaporated into the air, then changed back to water on the glass walls and came back to dirt. The plant never had too little water!
But not only plants that like water can be held in such containers. It depends on amout of water: a little of it and you can keep cacti inside. Some more and you can keep some flowers. Even mor and you can plant mangos, lemons and other exotic plants inside.