Vee-21™ by Kadon

This sweet and innocent-looking set of 21 V-shaped trominoes (3 squares each) can entertain a four-year-old with simple patterns, yet infuriate a grown-up expert with tough challenges. Math professor Norton Starr originally commissioned this set to demonstrate proof of a theorem—that an 8x8 grid can be filled with V-trominoes no matter where you leave the empty space. We coupled this puzzle with a color-separation feature, then added an original concept by Oriel Maximé, of filling the Vs on grids around strategically placed barricades. The 28 "Bends" layouts cover four levels, from Easy to Expert. Other challenges and game rules are included. A feature discovered later and added in the second edition of the book is a pretty alphabet. All acrylic, 7 tiles each of 3 luminous transparent colors in 7" tray with engraved grid lines. You can choose from three color combinations. Ages 4 to adult, 1 or 2 players.