Codeword Unlimited +, originally $1.19, is today's free app. It's a grid of words, similar to a normal crossword, but with no clues, rather a few initially provided letters, and a number in each square in the gird to indicate its (as yet unknown) letter. All boxes with the same number must use the same letter. From that information and the few initially provided letters it is possible to work out which words will fit in the boxes. Hence you are decoding the numbers into letters to reveal the hidden words. It's a good brain tease. Give it a try.