This lead-in to the "argument from lack of imagination" is another crazymaker. Its corollary is also useful, as in
1 I can imagine it, therefore it's possible.
2 If it's possible, it will eventually happen.
3 If it sometimes happens, it's true.
Ask your students what is wrong with this argument. By the time they're done there should be a whole list of wrongnesses. On the other hand, your entire class may be abducted by the next passing saucer... "It's an ill wind..."
Good luck and Hello to my old friends at PhACT.