Shrapnel,Named after Henry Shrapnel an English artillery engineer who invented an anti personel shell that shattered into metal fragments on detonation.
because metal is seemed hard and unbreakable, this is the image that the artists of that era where trying to portray. That no one could hurt them, they wouldn't go away without a fight, Hope this...
The (electromagnetic ) radiation they emit is short or micro wavelength ( 2.45 GHz frequency) which causes resonance in water molecules, resulting in rapid heating.