Why does it hurt the next day after doing significant exercise?

It hurts the next day after doing significant exercise because your muscles are working hard and getting tired.

When you run, jump, or lift heavy things, your muscles are like tiny engines inside your body. They go vroom-vroom to help you move. But just like a toy car that runs out of batteries, your muscles need time to rest and recover.

After the exercise is done, your muscles start to feel tired, kind of like how you feel after eating too much candy. They get little tiny tears in them, which makes them sore. These tiny tears are like small scratches on your skin, they don’t hurt right away, but later, when you sit down or move around, they remind you that you were working really hard.

That’s why the next day, after playing soccer or climbing a big hill, it feels like your legs are made of jelly, and that’s just your muscles telling you, “Hey, thanks for helping me today!”