Here is simple one, you have two candles of different length. You light both of them and cover them with glass cover of some sort (doesn't really matter what the cover is). Which one will die out first, tall candle or short one? It's easy to just do this experiment and and see it on practice, however thinking about what will happen is interesting and can lead astray.
Let us know what you think!
Note: Comments may have spoilers! Come up with solution on your own before reading them.