Square Root Of 169 Funny 13 Years Old 13th Birthday T-Shirt Solid colors: 100% Cotton; Heather Grey: 90% Cotton, 10% Polyester; All Other Heathers: 50% Cotton, 50% Polyester Imported Machine wash cold with like colors, dry low heat Math humor birthday shirt Perfect for any 13 year old Lightweight, Classic fit, Double-needle sleeve and bottom hem