Here's why:

* Mole: A mole is a unit of measurement in chemistry. It's defined as the amount of substance that contains as many elementary entities (atoms, molecules, ions, etc.) as there are atoms in 12 grams of carbon-12.

* Avogadro's Number: This number, approximately 6.022 x 10^23, represents the number of entities in one mole of any substance.

* Carbon's Atomic Mass: Carbon's atomic mass is approximately 12.011 atomic mass units (amu). This means that one carbon atom has a mass of about 12.011 amu.

Therefore, a mole of carbon contains 6.022 x 10^23 carbon atoms, which collectively weigh 12.011 grams.