Using masks in Photoshop is an essential skills to master. Masking ensures that you don't alter any of the actual pixels in your layer -- just hide or show them as you need. I wanted students to create a scene that involved multiple pictures of themselves interacting in some way. Here's the example I started them with:
The image that you see is actually comprised of 4 separate pictures -- the recycle bin alone, one for each hand and then one without the bin but with my head in just about the right place. This technique lets you create some really cool composite images. As always, I provided students with some guidelines and a few technical requirements. How to accomplish the task was entirely up to them (trying to encourage divergent thinking).
Here's what they created: