What does it mean for Satan to "sift" someone ? ? ?

Luk 22:31,32
And the Lord said, Simon, Simon, behold, Satan hath desired [to have] you, that he may sift [you] as wheat:
But I have prayed for thee, that thy faith fail not: and when thou art converted, strengthen thy brethren.

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Misery induces despair, pride induces presumption. The Incarnation shows man the
greatness of his misery by the greatness of the remedy which he required.
~ Pascal, Pensees 526