Pain.
PAIN.
Pain is good.
It's how you know you've reached your limit.
It's how you know where your limit is - and push past it.
It's how you know you've tried hard enough - and how to try harder next time.
Pain is good.
Even when you kick and scream and cry trying to hold it back.
The next time you'll still be laughing when you reach that point - and find a new point when you can't help but cringe.
And raise the bar one notch up.
Pain is good.