Children investigate volume as the amount of space an object takes up in 3 dimensions.
Children investigate capacity as the volume a container can hold when it is filled to capacity.
They use multiple uniform informal units to measure the volume and capacity of objects and containers with flat surfaces and straight lines, by packing in rows and layers, explaining why cubes are the best shape.
They measure the volume and capacity of containers with curved surfaces and curved lines by filling with liquid.
They relate the number of units to the size of the unit, and the size of the object or container.