A pentomino is a shape made of five squares joined edge-to-edge. There are 12 distinct ones, named after letters: F, I, L, N, P, T, U, V, W, X, Y, Z.
All 12 pentominoes together cover 60 squares — exactly enough to fill a 6×10, 5×12, 4×15 or 3×20 rectangle. The 8×8 square minus a 2×2 hole works too!