A terrarium is a self-sustaining ecosystem. Strictly speaking, a terrarium refers to plants being kept in an enclosed glass container. However, some has preferred to grow an open terrarium (which should really be called a table-top garden instead).
I've used this presentation to conduct a terrarium making workshop for young adults and have guided young children (1-to-1) to make their own terrarium. In the presentation, I've used an open glass bowl as an example to show the steps, which would be