Inkredibles: Launch into Maths for Curious Kids Invisible Ink Activity Book is packed full of awesome space-themed educational activities, including numeral stars, bingo, number pyramids, mazes, and more! Young number crunchers and maths enthusiasts will blast off into a whole new world of learning with Launch into Maths. The Invisible Ink pen can be used to answer puzzles and reveal the hidden answers, which makes learning a ton of fun!