Weight is how heavy a substance is. Weight is determined using a scale. In kitchens in the United States there are two units of measure most commonly used to express weight: pounds and ounces.

Many people believe that 1 cup of an ingredient always weighs 8 ounces, no matter what is in the cup. In truth, only 1 cup of water (or water-like liquids) will assuredly weigh 8 ounces.

If something other than water or a water-like substance is measured in a cup, it will still equal 8 fluid ounces, but it will not necessarily weigh eight ounces.

This confusion comes from not distinguishing the term ounce from the term fluid ounce. An ounce is something that you weigh, and a fluid ounce is a volume measure.