"Fartlek"

       You pop into a chair in an orthopedic physician's outer office waiting for her to see your wife about a foot problem.  You pick up a running magazine because you jog, and you come across a coach's article regarding training for marathons and become puzzled and smile when you read that the coach says he employs fartleks for his runners.

       Did I read that right?  Yes.  I did.  What in the world is a fartlek?

       And then you learn that a fartlek is a training device, especially useful for long-distance runners:  short bursts of fast running with slower paced passages in between.  The OED tells me later the two syllables of the word mean "speed" and "play" respectively, in Swedish.  
       Meanwhile, the English speaker, conjuring up a different "short burst,"  cannot refrain from smiling :).