A bright red-and-blue striped sock has lost its mate on laundry day. Shown side-by-side with four socks that are similar but not quite the same, a short rhyming text makes simple comparisons and contrasts. Finally a puppy solves the mystery when she drags the sock to her basket where the matching sock resides. Ehlert's flat, boldly colored illustrations enhance the concept throughout by making the differences and similarities easy for young readers to see right away. (Ages 4-6)