"Then the cat in the box would have died."
(Admittedly, this only works with people who have a handle on quantum physics.)
Does that refer to Schrödinger's cat paradox where one can create an immortal cat, that is to say that it is neither alive nor dead?
The cat isn't immortal; it is simply fluctuating between life and death (in a way its alive and dead at the same time), and when someone opens the box and views the cat it forces it to settle into life or death.
Dont worry, quantum physics gets a lot stranger than this
About the "but what if I chose the other one," I highly reccomend the sankey work. The effect is that you write a prediction of a card (i think? havent watched that one in ages) on a piece of paper, and you get the prediction right; taking your prediction and crumpling it up . You talk about how people always ask "what if" and you ask them to select another card. You then have them open up the paper and the prediction has changed to their second card.
The only problem with this is if you perform it for people you perform for regularly. They tend to ask to see other tricks like killer b done again, and you dont have an easy out.