Line up four glasses. Fill the first glass 1/4 of the way up with water. Fill the next glass 1/2 the way up, and the next one about 3/4 of the way. Fill the last glass all the way up with water.
Now tap the side of each glass with a spoon. Do you notice how each one sounds? What's different about each one is its "pitch".
Vibrations created by tapping the water glasses cause the different pitches. When the glasses are full, the vibrations have more water to travel through. This slows down the vibrations--and creates a lower pitch.
Pearson is taking advantage of this musical concept with his grain sorter. Wheat kernels that have been eaten through by insects create a different sound and pitch when pinging against a steel plate than those that are whole and intact.
Now, see if you can tap out a tune with the glasses! (You may have to adjust your water levels a bit.)