Capacity: This refers to the maximum amount that something can contain, often used interchangeably with 'volume'.
Cubic Meter (m^3): This is the standard unit for volume in the International System of Units (SI). It's like measuring how many standard-sized boxes (each 1 meter on all sides) you could fit inside your backpack.
Displacement: In terms of fluid mechanics, displacement refers to the volume of fluid displaced by an object when it's submerged. Imagine if your backpack was waterproof and you dunked it underwater - displacement would be how much water gets pushed out of the way.