I like the library-style detector idea... to nail the fiendish little perpetrators. Sirens, and rotating lights are definitely in order! Clifford's point about visitors not wanting to disable the unit is very interesting... This one puzzle that I'm working on now, "The Nine Fire Station Problem", uses these nine blocks to cover a six by six grid. Each "fire station" serves the 4 adjacent(not diagonal) squares(also, one corner is a "park"). Anyway, when we prototyped it, only about 1/2 the people could get it with nine blocks, and someone suggested to have ten blocks there so that the visitor could always get it right. In the copy, they would then ask them: "Can you do it with nine", making it an exercise in economy. I don't like that idea. I think that there should only be nine blocks, and if you can't get it, tough. Not to worry! According to Clifford's theory, the tenth superfluous block would probably (*hopefully*) get stolen! ??? -Jan [log in to unmask] (516)324-2683