Gather the materials you will need to conduct the experiment. The key to proving that the medium in which the flower is planted makes the difference in its growth is making that medium the only variable in the experiment. All other factors, such as the amount of medium, amount of water received and seed type, must remain constant. You will need two 8-ounce cups, potting soil, sand, water, a measuring cup, ruler, flower seeds and two calendars.
Use the measuring cup to fill one 8-ounce cup with 6 ounces of soil. Fill the other cup with 6 ounces of sand. Push one flower seed into each cup halfway through the medium. Water each with 1/4-quarter cup water. Place the cups side by side on a window ledge that receives consistent sunlight throughout the day. Water each cup with 1/4-quarter cup water daily for one week. Continue watering the plants for another week but give each plant a 1/4-cup water every other day.
Assign a calendar to each cup: soil and sand. Record the amount of water given to each cup each day. These numbers should be identical. Use the ruler to measure each flower's height when it begins to sprout and grow. Record these numbers on the calendar as well. Write down notes about any visual observations made, such as the dryness of the soil, the color of the flower or the number of petals each one has.
At the end of the two-week experiment, compare the overall height of the flowers. Also compare the rate of growth of each of the flowers, which can be seen by looking at the data from the entire two weeks. The flower which is the tallest and healthiest should have sprouted from the soil cup.