Metal

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.

However, metal is not completely unaffected by the balls; It can be pushed if it or its glass is not being held by any strings. That can be useful for saving balls or saving yourself from being hit.

It also acts as a support for most obstacles. Look closely to where a glass pane or bar is anchored. There you will find metal.