To make a terrarium, choose a glass container with an opening wide enough for your hand. Gently add an inch or two of washed, fine gravel. Top gravel with a thin layer of activated aquarium carbon. (You'll find both items at your local pet store.) Next, add moistened potting soil, and you'll be ready to plant. Create a collection of plants, or showcase just one. Good choices include ferns, succulents, mosses, miniature moth orchids, African violets, and kalanchoes.