Adobe PhotoShop - Sample Pixels From More Than Just The Active Layer (Photoshop 7/CS)
You can sample pixels from layers other than the active one. To do so, select the Magic Wand, Smudge, Blur, Sharpen, Paint Bucket, or Clone Stamp tool from the Toolbox. Then, select the Use All Layers check box on the tool options bar. The tool you chose samples pixels from all visible layers, not just the active layer.