So simple. No one seemed to explain quite how it works. This is a sealed unit so no liquid/juice actually goes into the instrument. There's a flip-up piece of plastic on the sloped part in the front of the unit. You simply flip the plastic part up and reveal a sloped piece of glass underneath. You put a few drops of juice on the glass then flip the plastic flap back over the glass, squishing the juice between the plastic and the glass. Make sure there are no bubbles between the glass and plastic and now you have a very thin sample (much like you would do with a microscope with two pieces of glass). When you look through the hole in the back, you'll see the scales. Above the reading on the scale is colorless while the part below the reading is blue. The dividing line between the blue part and colorless is your reading on the scale. Brix is on one side while specific gravity is on the other. When you're done, flip the plastic up, wipe off the glass and plastic then you're ready for another sample. Grape by grape, bunch by bunch - you'll use it over & over!