We offer a solution and interpretation of a curious old Sangaku wooden tablet puzzle involving three mutually tangent circles C0, C1, and C2, together with an isosceles triangle T. Let C0 be the largest circle, and let d denote a diameter of C0. Along d lie both the center of C1 and the base of T. The endpoints of the base are denoted by A and B, where A ∈ C0 and B ∈ C1. The problem asks: Where is the center Q of the third circle C2, which is tangent to C0, C1, and the sides of T? Surprisingly, the point Q always lies on the line perpendicular to d at B, regardless of the position of B along d.

This poster is for a general audience.