Here's The Joke

I was working in a scrap yard during summer vacation at
engineering university. I worked repairing construction
equipment.

One afternoon, I was taking apart a piling hammer that had
some very large bolts holding it together. One of the nuts
had corroded on to the bolt; to free it I started heating
the nut with an oxyacetylene torch. As I was doing this,
one of the dimmest apprentices I have ever known came along.

He asked me what I was doing. I patiently explained that if
I heated the nut it would grow larger and release its grip
on the bolt so I could then remove it.

"So things get larger when they get hot, do they?" he asked.

"Yes," I said, "that's why days are longer in summer and
shorter in winter."

There was a long pause, then his face cleared. "You know, I
always wondered about that," he said.