TEN-YEN™ by Kadon

10 acrylic pieces include all the shapes of 1, 2, and 3 squares in size, plus 3 each of the 4s and 5s. The three colors all have the same area, but it's tough to get them all to make the same shape—there are exactly 17 ways to do it. Or how about the four quads problem—to make 4 identical shapes of 9 units each has only 3 possibilities. Or try the enclosure challenge:  Each color can be enclosed by the other two. What is the shortest perimeter? How about the longest perimeter? Filling the 6x6 pattern in the 5.5" tray with color-matching and non-matching is just a small part of the countless other figures to solve, both easy and tough, including a pretty alphabet. Great for one to four players, ages 6 to adult.