My students and I had just concluded a great discussion about idioms.  We explored many examples, giggled over what a literal interpretations might look like--"getting someone's goat," "stuck between a rock and a hard place," "being 'in hot water,'" something costing "an arm and a leg," "raining cats and dogs," etc.  The students had just settled into their assignment--to draw a picture to illustrate an idiom of their choice--when a boy exclaimed from his seat, "Oh, I thought of another one!"  
"Oh, what is it?"  I asked.  
"Some guy is "light in his shoes!"  the boy announced, innocently.  
Uh, oh.  "Oh!  You mean someone is "light on his feet!" I replied, thinking as quickly as I could.  
I could tell by the boy's expression that this did not quite fit what he had in mind, but he opted not to pursue it, for which I am grateful.  I had no particular desire to touch that one... not even with a ten-foot pole.