Second wind

When you run or do any exercise, your body needs lots of oxygen to keep going. After a while, you might start to feel tired and out of breath. But, if you keep going, your body starts to figure out a better way to use the oxygen it has. This is called a "second wind". It's like your body saying "oh wait, we need to keep going, let's use the oxygen better!" So, your breathing may slow down, and it may not be as hard to keep exercising. The second wind can help you go further and feel less tired while you do it!