Grout is the wet mix you push into the little gaps between tiles on a wall or floor. It is sand, water, and cement or lime. When it dries, it holds the tiles and stops water sneaking behind them. You can also use it as a verb: you grout the tiles. It is a home-and-building word; you hear it in hardware shops and from people fixing bathrooms, not in school stories. The lines you see between the squares are the grout — they start pale and often go grey if nobody cleans them. Though actually, grout is not the glue that sticks the tile to the wall. That glue goes underneath. Grout is only for the gaps. Say it like out, not like root.