Students collect weed seeds from three varieties of weeds (knapweed, thistle, or toadflax, for example they may choose others in addition to the spotted knapweed.) They sterilize soil for thirty containers, plant seeds in sterile soil, and mix vinegar with distilled water and label the container (25 mL vinegar and add distilled water to the 1,000 mL mark so the total volume equals 1,000 mL or 1 L).