The Cannon
Spoiler: this page reveals the second secret.
If you are the grown-up in a family, play first — the child gets to show you. If you came here from a search about the xiangqi cannon, read on.
Reveal the rule
The cannon comes from Chinese chess (xiangqi). It slides straight like a rook — but it cannot capture that way. To capture, it must jump over exactly one piece (the screen, any colour, even its own king) and land on the enemy piece beyond.
That is why a grown-up gets it wrong: the piece in the middle looks like a blocker. For the cannon it is the thing that makes the shot possible. Zero pieces in between — no capture. Two — no capture. Exactly one — shot.
In the game
The child places a guard on the line. The grown-up sees a “blocked” cannon and tries something else. Then the grown-up moves that guard away — and the child must find the new way out. On our 5×5 board the cannon’s rule is checked against a separate chess oracle, so what the child is shown is the real xiangqi rule, not an approximation.