When someone cheats, sometimes you do not need to accuse them. You only need to force the cheater to accept the consequences of their own lie.
The King's Two Stones
Riddle statement
A king publicly claims that he has put one white stone and one black stone into a bag.
A prisoner must draw one at random: if it is white, he will go free; if it is black, he will die.
But the prisoner sees that the king cheats and puts two black stones into the bag. He cannot accuse him directly.
What can he do to save himself?
Show solution
Solution
The prisoner draws a stone and, before anyone sees its color, drops it among many indistinguishable stones in the courtyard.
It is no longer possible to identify which stone he chose. To determine its color, the only option is to look at the stone that remains in the bag.
The remaining stone is black. Since the king had publicly claimed that there was one white stone and one black stone, he must maintain that the lost stone was the white one. Denying that would amount to admitting the cheat.
Therefore, the prisoner goes free.
He does not need to swallow anything. The essential move is to make the chosen stone unrecoverable and use the remaining stone as indirect proof.
Answer: he should make his stone disappear without showing it and ask that the remaining stone be examined.