I'm not sure who Grandpa is but my grandfather never taught me this game and if he had I probably wouldn't have been very impressed. I had better things to do with my grandfather, like watch rugby and play football.
Grandpa's Challenge is a very simple puzzle which tests logic.
The aim is to move the large yellow square to the bottom left corner. That really is it.
Just drag the other pieces in a direction of your choice, clearing the area, and get the yellow square to the bottom. It sounds simple because it is.
I didn't find it much of a challenge unfortunately. The fewer moves you make the more points you get and for a while it is fun. But it does get boring rather quickly.