Crossing & Weighing Puzzles
📋 What it is
Crossing, pouring, and weighing puzzles have strict RULES; the trick is respecting every constraint each step.
🗣️ Coach says
These classic puzzles are all about constraints. Fox-goose-grain (what can’t be left together), water-jugs (only GCD amounts), coin-weighing (split into groups). List the rules FIRST, then never break one — even if it means a step backwards.
🧠 Memory hook
List the rules first, then never break one — even a backward step keeps you legal.
😂 Giggle
Why did the estimate and the exact answer high-five?
Because the reckoner checked one against the other!
😲 Whoa!
The “weigh 12 coins to find the fake in 3 weighings” puzzle is so elegant it’s used in real job interviews at top tech companies to test problem-solving.
✅ Quick check: In fox-goose-grain, why might you carry something BACK across the river?
Say your answer out loud first — then reveal.
To avoid leaving a forbidden pair alone — a backward step is sometimes the only way to keep every rule.
Constraint puzzles often need “backward” moves to stay legal.
🧪 Try it! (2 minutes)
Solve fox-goose-grain by writing the “can’t be alone together” pairs FIRST, then plan crossings.
🔎 How did you work this?