I inherited a stop/bounce dart when I arrived at my school in 1986. You arrange the dart as a pendulum and swing it into a wood block. Back in those days, you ran the unadorned nail point into the block for the stop, and attached a rubber stopped to the nail point for the bounce.
Since then, the advent of "happy" and "sad" balls improved this demo. And happy/sad pendula/mallets have been commercially available (help me out with a link if you know one).
Arrange the dart so that it barely knocks over the block using the happy ball on the tip. Then switch to the sad ball and allow for predictions and arguments. Then proceed with the reveal.
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