Scagliola is like play-dough for grown-ups. Imagine you are making a cake and need to mix flour, butter, sugar, and eggs together to make the batter. Scagliola is a mixture made of things like plaster, glue, and marble dust.
Just like how you can mix different food colors to make a rainbow cake, scagliola can come in lots of different colors.
Instead of using this mixture to make a cake, people use it to make beautiful decorations for buildings. They start by making a big sculpture or design, and then cover it with the scagliola mixture. Once it dries, it looks just like the real thing!
So scagliola is basically like a grown-up version of play-dough used to make fancy decorations for buildings.