When you go outside to play soccer or run around, you use your muscles a lot. But sometimes, if you don't warm up properly or if you overuse your muscles, they can get hurt. That's called a strain!
Think of your muscles like silly putty. When you pull on silly putty, it stretches and gets longer. But if you pull too hard, the silly putty can break. The same thing can happen to your muscles when you strain them. If you pull or overuse them too much, they can get torn or damaged.
Strains can happen in different places on your body - like your legs, arms or back. When you get a strain, you might feel pain, swelling, or a stiff/tight feeling in the area that’s hurt. It's important to rest your muscles and not play too much until they feel better. That way, your body can heal and you can get back to playing again soon!