Mass
Mass is basically a count of how many "atoms" are hanging out inside something. Think of it this way: that 1-kilogram chunk of steel on the scale is packed with so much matter it’s like a sold-out stadium, while a feather is just a couple of people wandering around an empty park.
Even though they might take up the same space, the steel is definitely winning the "how much stuff can I cram in here?" contest.
Even though they might take up the same space, the steel is definitely winning the "how much stuff can I cram in here?" contest.