A good one i done on the boss was, i was drilling a hole down the centre of a bolt, i cant remember why but any way it was to hot to hold so i was rolling it about in my hand so it didnt get to warm and the boss saw me and said let me see that, i couldnt resist so i handed it to him and he held it up in front of him in the light to see it, it took about a second before he felt it was to hot but by the then it was to late, he threw it down and ate me while i was bent over in fits of laughter holding my chest trying to breathe. He went out and his son came in looking to know what happened. I told him what i done then i drilled the bolt a bit futher, it was again to hot to hold without moving it, i was admiring how straight it was when i told his son how perfect it was he wanted to see it, so i gave it to his son and i got him too just after telling what i had done to his father. You could do it with anything hot, even a headlight bulb. Hold it up like your looking to see if its okay but hold it by the steel then ask him to see if he can see it but give it to him by the glass.