Instead of sending out a card filled with Christmas cheer, Britain’s spy agency GCHQ sent out a card, asking to see if people solve a challenging puzzle.
The puzzle consists of a grid featuring select blocks that are shaded in along with a series of numbers.
According to GCHQ:
“In this type of grid-shading puzzle, each square is either black or white. Some of the black squares have already been filled in for you. Each row or column is labeled with a string of numbers. The numbers indicate the length of all consecutive runs of black squares, and are displayed in the order that the runs appear in that line. For example, a label ‘2 1 6’ indicates sets of two, one and six black squares, each of which will have at least one white square separating them.”
