Look for pairs of objects that, when stretched, deformed, and made bigger/smaller, could be turned into the same shape. Think about what limitations not being able to join edges would cause, specifically, what kind of construction would you not be able to turn into a blob if you couldn't join edges?

Look at the Wiki article for phrases in the question, they've got a demo of how this works, amusingly using some of the objects from the question, once you've seen the demo the other two pairs should be easy.