system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what happens inside the body when athletes get a “2nd wind”?
gpt
Your body doesn't want to exercise.  Your body gets whiny and says, "We're tired, let's go lie down under a tree and take a nap."

Then you push through and your body says "Fine, if you're going to be that way, let's go nuts!  Let's really kick those glycogen fires up, let's throw some fat on there too."

Now you have a timer running.  That time is your glycogen stores.  You only have so much until your muscles are out for a while.  That means you can only burn fat which is piss-poor for intense energy.  

That's where you "Bonk".  You feel it when your muscles kind of start to feel limp and week, you're a little nauseous, and you could eat all the pancakes in the world.  It's the pancake thing that gets me.  

Now you're done with anything like sprinting or high intensity for a while.  You can keep pushing but it takes time for your muscles to rebuild glycogen.  Eating lots of carbs before and during the workout helps.