That's an amazing practical joke.

When I was in nuclear power school we played an amazing prank on our instructor:

Dip both ends of a slightly used piece of chalk in superglue. It's invisible but makes it hard as porcelain. It looks like chalk but doesn't write at all.

The guy gets through half an invisible weird before he notices nothing coming out.

He stops and slowly goes to another section of the board an tried to draw a 1" mark (also invisible).

At this point the 4-5 of us in on the joke were giggling pretty good so he knew it was a prank. I can't remember if we fessed up or he figured it out he just broke it in half and got to work.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)