When a star is on the way to converting all of its carbon and oxygen atoms into iron, calcium is one kind of atom it makes along the way. One atom of calcium has 20 protons and 20 electrons (and 20 neutrons). Calcium is a light kind of metal, not as strong as iron, though it is harder than lead.
After the star is done changing its atoms to iron, it explodes, shooting the calcium atoms out into a new nebula. Many of the calcium atoms in our nebula got made into part of the Earth. So many calcium atoms reached Earth, in fact, that calcium is the fifth most common element in the Earth’s crust.
Most of the calcium on Earth is mixed with carbon to make molecules of calcium carbonate. That’s what sedimentary rocks like limestone and chalk are made out of. Then we burn limestone to make cement and concrete.
But calcium is also one of the main atoms involved in making living creatures. Sponges and other animals make their hard structure out of calcium. All chordates make their bones and eggshells out of calcium.
People need calcium too: our bones are mostly made of calcium. That’s why it’s important to eat foods that have calcium in them. Some people drink milk or eat cheese or yogurt. Others eat tofu, beans, almonds, and kale to get enough calcium. Kids who don’t get enough calcium can get rickets, where your leg bones are weak and curved.