system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If 8 ounces is a cup, why does my 16 oz bag of shredded cheese say 4 cups?
gpt
Ounces are a unit of weight, while cups are a unit of volume. Different materials have different densities (the ratio of weight to volume). For example, pillow stuffing has far fewer ounces per cup than steel.

Adding some confusion to the issue, the "fluid ounce" is a unit of volume, equal to the volume of one weight-ounce of water. Thus, a cup (eight fl.oz.) of water weighs eight weight-ounces, but a cup of cheese (also eight fl.oz.) may weigh significantly less.