Chalk is a soft, white rock made mostly of ancient sea creatures called plankton that lived millions of years ago. When these tiny organisms died, their shells piled up on the ocean floor and eventually compressed into thick layers of chalk. The famous White Cliffs of Dover in England are made entirely of chalk.

People use chalk in two main ways. Teachers write on blackboards with sticks of processed chalk, which leaves a powdery white mark that wipes away easily. The squeaky sound chalk makes on a board happens when it vibrates against the surface. Athletes like gymnasts and rock climbers rub chalk powder on their hands to absorb sweat and improve their grip. Baseball players use it in the batter's box, and pool players use it on their cue tips.

When something is described as being as different as chalk and cheese, it means two things look similar but are completely different. If you chalk something up to experience, you're accepting a mistake or loss as a learning opportunity. When a team chalks up a win, they've added another victory to their record.

As a verb, to chalk something is to mark or draw it with chalk, like chalking a hopscotch grid on the sidewalk.