What Makes a Puzzle Hard? Search, Outcomes and Hidden Rules
Four things make a puzzle hard: a huge search, tests that reveal little, long solutions and hidden constraints. What the research shows for each.
Riddle10 blog
Articles about coin weighing
How to Solve Coin Weighing Puzzles, Step by Step
Split the coins into three groups, not two. A clear method for balance scale puzzles, from 9 coins in 2 weighings to the classic 12-coin puzzle.