Grandpa teaches his grandson how to calculate the area of his farm and the capacity of his grain silos. He explains that to find the area of the farm, you calculate the length multiplied by the breadth. To find the capacity of a silo, you use the formula πr2h, where r is the radius and h is the height. The grandson thanks Grandpa for teaching him these useful formulas and says he will ask his teacher to learn more when he returns home.