Capacity measures how much a container holds. Common units are cups, pints, quarts, gallons (customary) and milliliters, liters (metric). Pouring and comparing helps students develop intuition for capacity.
You already have a sense of what capacity means — it is how much a container can hold. Now we put precise numbers on that idea by using standard units. Without standard units, "a big cup" or "a small bottle" means something different to every person. Units let everyone measure and communicate capacity in a way that is consistent and comparable.
In the customary system, the four main units are the cup, pint, quart, and gallon. They nest inside each other: 2 cups make 1 pint, 2 pints make 1 quart, and 4 quarts make 1 gallon. A good way to build intuition is to connect these to familiar containers: a standard drinking glass holds about 1 cup; a small milk carton at lunch is about 1 pint; a large sports drink bottle might be close to 1 quart; a large milk jug from the grocery store is 1 gallon. These anchors help you estimate before you measure.
In the metric system, the two main units at this level are the milliliter (mL) and the liter (L). One liter equals 1,000 milliliters. A standard water bottle is about 500 mL, or half a liter. Milliliters are useful for small amounts — medicine is often measured in mL. Liters are practical for larger amounts like beverages or cooking.
Measuring capacity works best with hands-on practice: fill a cup with water and pour it into a pint container to see that you need two cups. This physical experience builds the lasting intuition that no amount of memorizing conversion numbers can replace. When you encounter measurement problems later — including converting between units — these concrete mental images of containers give you something real to anchor your reasoning.