A few days ago I had to give my students a very informal explanation of the notion of logical possibility: an entity or a state of affairs is 'logically possible' if its description does not involve a logical contradiction. As usual, I started with a trivial example. I said:
"I'll tell you the beginning of a story - just a couple of sentences - and then I'll stop and ask you if I should continue because you accept the beginning as describing something that is possible. So, yesterday I was at a garage sale where I saw a coffee table in the shape of a square circle, i.e., the shape that is both a genuine square and an honest-to-goodness circle. I bought this coffee table and brought it home."