Running - the thought of it probably sends shivers down your spine.
But some people love pounding the pavement for a reason some of us may never understand.
While many of us would rather participate in activities that leave us less blotchy and panting for breath, some people actually feel really good about running.
We're not sure why – but apparently science does.
Nanotech scientist Michelle Dickinson joined Paul Henry in studio this morning to explain.
Apparently, it's not to do with endorphins – but the urge to run dates all the way back to our caveman days.
Watch the video for the full interview.