Surely you have heard the question "What weighs more, a kg of lead or a kg of cotton? This question is used by teachers to teach young students a clear difference between mass, volume and density. A kilogram is a unit that is used to quantify masses and therefore both kilograms will be equal; what really varies is the volume, since, a kilogram of lead could easily fit in our hands, while a kilogram of cotton could be 10 pillows, this is due to their densities that are very different.

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