Water

One of the beauties of the metric system is that it has terms for volume—“milliliters” and “liters”—and different terms for weight—“grams” and “kilograms.” The metric system is based on water, that is, the volume value for water is equal to the weight value (100 milliliters are equal to 100 grams). Our avoirdupois system is less elegant and somewhat confusing because not only does the word “ounce” refer to both volume and weight, 1 fluid ounce of water does not equal 1 ounce in weight; it equals 1.042 ounces. So one 8-ounce (in volume) cup of water is actually equal to 8.33 ounces (in weight). Webster’s American Dictionary defines water with an equation: 1 gallon equals 8.337 pounds (133.392 ounces). This divides down to 1 cup equals 8.337 ounces/236.35 grams.