Never let it be said the The 3:16 Show can't be both entertaining AND educational! It's rarely either of those things, of course, but it can be. The potential's there. Hit the play button above, and a radioactive vial will smash and kill a cat! Or maybe it won't. Or maybe both.
I have no idea how well I managed to explain one of the most baffling aspects of one of the most complicated scientific fields in just over three minutes, but the implications of Wheeler's delayed choice experiment really are as mind-shattering as they sound. Not only that, but this isn't just purely hypothetical science (or hippy-dippy quantum woo); this is an experiment they have actually conducted in the real universe, and the results consistently point towards one conclusion - the distant past of the universe changes the instant a human looks at it.
Makes you wonder what the universe is playing at, really.
Here's the wikipedia entry (see the 'cosmic interferometer' section), or check out this Popular Mechanics article which is about as accessible as you can get with this kind of stuff, which is not at all.