Mini Terrarium Materials: 2 clear plastic cups potting soil transparent tape radish seed or grass seed Method. soak enough soil to fill on cup half full. Sprinkle the seeds on top. Invert the other cup pover the top and tape the seams. Plae in a warm, sunny plae and wait for the seeds to sprout. A terrarium is like a tiny world. The top cup is like the sky. It traps the evaprating water from the soil and recycles it over and over. .