Mark Twain - Anger Is an Acid - Famous Person Classroom Poster Mark Twain - Anger Is an Acid - Famous Person Classroom Poster 12" x 18" Poster on High Quality heavy 80lb satin cover paper - durable and can stand up to all kinds of abuse and it won't pucker and wrinkle like others do. This new poster is a PosterEnvy EXCLUSIVE! That means you won't find it anywhere else! We ship orders out FAST (usually with 24 hours) in heavy duty cardboard tubes! PosterEnvy has sold thousands of unique educational posters to thousands of happy customers around the world!