I teach a high school class in which we spend a couple weeks in powerpoint. We're making graphs, and every now and then a student will get a gray box around the graph and it more or less locks up the program. If they try to delete it or close the slide, it says that the chart contains an embedded object and that you must close the embedded object in order to close the slide (can't remember the exact wording). But I can't figure out how I'm supposed to close the embedded object.

This has never happened to me when I'm using PowerPoint. What are they doing to get this embedded object in the first place (I'm sure they're not going INSERT | OBJECT) , and how do you close it.