The Grasshopper
Spoiler: this page reveals the third 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 grasshopper piece, read on.
Reveal the rule
The grasshopper is a classic fairy chess piece (invented for chess problems in the 1910s). It moves along queen lines — but only by hopping over the first piece it meets and landing on the square immediately behind it. No piece to hop — no move in that direction.
Its obstacle is its springboard. A grown-up reads the guard as a wall; the child knows it is the launch pad, and can predict exactly where the grasshopper will land.
In the game
Here the threat is a diagonal one (an enemy bishop). The child places the springboard guard; the grown-up tries to run with the king; the grasshopper hops the guard and captures the bishop. Then the grown-up moves the springboard — and the child finds the new way out. The rule is checked against a separate chess oracle.