Explain the process of Grading and Sieving

Grading: Grading consists of separating the sound kernels from chaff and impurities,and may be achieved by sieving or winnowing.
 
Sieving: Impurities are separated on the basis of their differences in size from the kernels. Hand sieves are usually used singly. The simpler machines will have two sieves: one with oversized holes (which retain large impurities and let the grain kernel pass through) and the other one with undersized holes (which retain the kernels but allow smaller impurities to pass through).