TIP: Histogram for Just One Layer

This is handy tip for anyone who makes collages, and needs to tweak the tone of different images on different layers. Here's the deal: By default Photoshop's Histogram palette displays the Histogram for the entire image (as if the file was flattened). However, you can change it so it display the Histogram for only the active layer. Just go to the Histogram palette, from the pop-up menu choose Expanded View, then change the Source from Entire image to Selected Layer.