Does caffeine help or hurt a growing plant? You plant mung beans in three pots and water them all with plain tap water for the first five days. After the seeds sprout, you switch one pot to a caffeine tablet solution and another to a coffee mixture. The third pot stays on plain water as your control. You measure the average plant height in each pot every day for ten days. The coffee-watered plants grow the tallest. The caffeine-watered plants grow the slowest, even slower than the plain water group. The surprise result shows that pure caffeine and brewed coffee affect plant growth in opposite ways.