Why did ancient sellers put a grass on their children's heads?

"Grass" means cheap, that is, this thing has no "value" (even unnecessary) to me, so it is sold.

Straw labels are also called dirt. This is a naturally growing thing, but when it is inserted into the items for sale or sale, it has the meaning of recognition. Mustard was used in different historical periods of ancient China society, from daily necessities to poor families who sold their children because of hunger and cold.

Cao Biao has been mentioned many times in Water Margin, the most famous one is about Yang Zhi selling knives: "Yang Zhi bought a treasure knife that day, inserted a Cao Biao and sold it on the market." The grass sign here has the function of advertising media.