Granny has to punish aliens with her cane for abducting people. Use your mouse to knock jewels off the UFO (I guess you didn't know aliens use jewels as parts of UFO's engines? well, you do now!). Grandpa will help you (yeah, he's actually going to help granny, not you. But since you control her...), he'll collect jewels and sell them (instantly, wow!).
P.S.
As for the carpal tunnel syndrome, you should be only concerned if your work involves the same repetitive wrist motion (and in this case the problem is the work, not the game). Otherwise the game will give you a nice exercise, since holding a mouse, our wrist tends to have an unnatural bend, harmful for the median nerve in static. So keep calm and carry on.