A sit-up is an exercise where you lie on your back with your knees bent, then use your stomach muscles to lift your upper body toward your knees before lowering yourself back down. During gym class, you might do sets of sit-ups to strengthen your core, which includes all the muscles around your middle that help you bend, twist, and stay balanced.

Sit-ups have been a standard fitness exercise for decades. Athletes use them to build the strength needed for sports like gymnastics, martial arts, and football. Soldiers often do sit-ups as part of their physical training. While some fitness experts now prefer exercises like planks or crunches (which are like gentler sit-ups where you don't come up as far), sit-ups remain popular because they're simple, require no equipment, and clearly show improvement as you get stronger.

The term can also describe suddenly paying attention, like when a teacher announces a surprise field trip and everyone sits up in their chairs. When something makes you sit up and take notice, it grabs your attention so powerfully that you become instantly alert.