How to explain undefined behavior to know-it-all newbies?
Solution 1:
Undefined means explicitly unreliable. Software should be reliable. You shouldn't have to say much else.
A frozen pond is a good example of an undefined walking surface. Just because you make it across once doesn't mean you should add the shortcut to your paper route, especially if you're planning for the four seasons.
Solution 2:
Two possibilities come to my mind:
You could ask them "just because you can drive on the motorway the opposite direction at midnight and survive, would you do it regularly?"
The more involved solution might be to set up a different compiler / run environment to show them how it fails spectacularly under different circumstances.
Solution 3:
"Congratulations, you've defined the behavior that compiler has for that operation. I'll expect the report on the behavior that the other 200 compilers that exist in the world exhibit to be on my desk by 10 AM tomorrow. Don't disappoint me now, your future looks promising!"