People who are naughty get lumps of coal for Christmas, but elves who are naughty have to work the coal mines.

Once there was a gang of elves that were so bad, they delayed Christmas by so far that we almost missed it.  The next year we caught them.  They were sent to the coal mines.  They worked day and night.  They were tired, and they would have run away, but there were two Yetis at the entrance keeping guard so that nothing could come out, and hardly anything came in.  So, they were plotting to make it look like the mines were exploding.  They were working on a plan, but when the Yeti's turned around to check their work, they looked busy.

This is what it was like working there.  You're mining all day and all night.  It's cold with no heaters.  You get one coat and a hat.  Most of the time, you worked all month long, with only 1 meal a day and no snacks.  If you disobeyed time was added to your sentence.  Their plan was to make some of the coals burn, so the Yetis would leave to get things to put the fires out.  Then they would escape before the Yetis could get back.  But if they don't think the entire mine is catching on fire, you don't want an angry Yeti yelling at you.  They are big, mean, and scary.  So most elves, just waited until their working in the mines was over.

But the plan worked, and the elves were back to trickering.  But when they were caught again, they got hurt.  Ever since then, they never tried to cheat people or trick them ever again.