100-cell calculation is invented to develop a fundamental mathematic capacity of elementary school students in Japan. There are 10 numbers in first row and 10 numbers in first column. And you just add two numbers from top and left 10×10=100 times like this.
It's devised for small kids but it's also good for adults to keep their brain younger and healthier. When you feel sleepy or tired, please try to make some calculation. Maybe your brain gets much clearer.