I know thy... tribulation... but... be thou faithful... and I will give thee a crown of life. (Revelation 2:9,10)
 
Thousands of years ago, farmers used a tool called a "tribulum" to separate the chaff from their wheat. Today our word "tribulation" is derived from the name of that tool. 
Just as the ancient farmers used a tribulum to separate the chaff from their wheat, God uses tribulations to separate the chaff from our lives.
Therefore, "think it not strange concerning the fiery trial which is come to try you" (1 Peter 4:12). “Knowing... that the trying of your faith worketh patience... that ye may be perfect and entire, wanting nothing” (James 1:3,4).

Afflictions, Refining Influence Of: The Tribulum
In the pictures of the ancient Roman method of threshing grain, one man is always seen stirring up the sheaves while another rides over them in a crude cart equipped with rollers instead of wheels. Sharp stones and rough bits of iron were attached to these cylinders to help separate the husks from the grain. This simple cart was called a tribulum -- from which we get our word "tribulation."

When great affliction comes to us, we often think of ourselves as being torn to pieces under the cruel pressures of adverse circumstances. Yet as no thresher ever yoked up his tribulum for the mere purpose of tearing up his sheaves but to disclose the precious grain, so our loving Savior never puts us under the pressure of sorrow and disappointment needlessly.