Why won't my eraser tool work when I am trying to make one object superimposed upon another partially transparent?

I have superimposed one object upon another. I want to make a portion of the top image transparent so that part of the object below visible. But when I select the eraser tool, it disappears when I move it to the place I want to erase. I can move any of the other tools in the tool bar to the location but not the eraser too. I have tried to look for some "lock" that might be accidentally activated but I can't find one. Any ideas?