Researchers want to test if a new detergent works better in warm or cold water for hand-washing clothes. To do this, they will use a randomized block design where they block, or separate, the clothes into categories based on expected differences in how they respond to temperature (e.g. cotton vs. wool). They will then randomly assign the detergent and water temperatures within each block to get more precise conclusions about how the detergent performs in each temperature for each type of fabric.