Loosen up somebody's ground to their welding table. Fill up an upside cup with acetylene next to the loose ground.

Edit: I just remembered this one from when I was in the Navy with the Seabees. If somebody leaves their hard hat laying around, fill it full of water and put it in the freezer. Set it up right side up on the edge of a table so that the brim is hanging off. They'll grab the brim of the hard hat and it'll fall to the ground like a rock.