Imagine you have two toy boxes, one filled with large, fluffy stuffed animals and one filled with small, hard toy cars. Both boxes can hold the same number of toys, but they feel very different when you pick them up. The fluffy stuffed animals take up more space and feel lighter, while the small toy cars are more compact and feel heavier.
Paper density works in a similar way. It measures how tightly packed the fibers in a sheet of paper are. If the fibers are packed closely together, the paper will be denser and feel heavier for its size. If the fibers are looser, the paper will be less dense and feel lighter for its size.
Paper density is measured in grams per cubic centimeter (g/cm³). This means that if you have a cube made of paper that is one centimeter on all sides, you can weigh it to see how many grams it has, and that gives you its density.
Different types of paper have different densities. For example, high-quality printer paper might have a density of around 0.75 g/cm³, while newsprint paper used for newspapers might only have a density of 0.3 g/cm³. This is why newspaper pages feel lighter and flimsier than printer paper, even though they can be the same size.
So, just like how different toys can take up the same amount of space but feel different when you hold them, different types of paper can have the same size but feel different weights and thicknesses because of their densities.