US Customary Units: Conquered?

Here we go:

Volume

• 2 pints = 1 quart
• 4 quarts = 1 gallon
• 16 fl. ounces = 1 pint
• 8 fl. ounces = 1 cup
• 2 cups = 1 pint
• 4 cups = 1 quart

Mass

• 16 ounces = 1 pound

...I think, anyway. Why is this so difficult?