To grow an avocado seed, first, remove the seed from the fruit and rinse it under cold water to remove any flesh. Pierce the seed with toothpicks and suspend it halfway in a glass of water, ensuring the bottom half is submerged. Place the glass in a warm, sunny spot, and change the water every few days. After a few weeks, roots will sprout, followed by a stem. Once the stem reaches around 6 inches, transplant it into soil, leaving the top half exposed. Keep the soil moist and place it in a sunny location to encourage growth.