Metal, unlike Glass is indestructible and is very dangerous.
Metal can only fall by destroying the glass connected to it. Hitting it with an Explosive Balls Power Up will usually destroy the glass conected to it.
Howhever, Metal is not completely unnafected by the balls : It can be pushed if its not being hold by any glass \ glass connected to it is on only one side. That can be useful for saving balls or saving yourself from getting hit.