Slag

Okay kiddo, so when people make things out of metal, like cars or bikes or even big buildings, they have to melt the metal so they can shape it into what they want. And when they melt the metal like that, little pieces of dirt and other things that were in the metal float to the top of the melted metal and form a kind of crust.

That crust is what we call slag. It's kind of like a scab on your knee after you scrape it - it's the body's way of protecting itself from further harm. In this case, the slag is protecting the melted metal from any more dirt or other things getting in there.

So once the metal is all melted and shaped how people want it, they have to get rid of the slag. They usually do this by letting the metal cool down and then scraping the slag off the top. Then they can use the metal for whatever they need it for!