Give your children strong refrigerator magnets. (This worked best with the 5&4 year olds.)
Tell them to go find things that are metal, demonstrate how the magnet sticks to metal, but not to wood or glass or plastic or you.
Admonish them that if they get something out to test it they have to put it back-if you skip this step, you will rue the day you ever read this.
Remember, as they are prancing about the room with their magnets stuck to forks and measuring cups and singing the impromptu “metal song”, which is annoying, but less annoying than, say, Barney; that they are learning something, happy and occupied and go do something quick before they get distracted!