Draw the Fischer projections of the four aldotetroses. Draw the D sugar on the left and its L isomer directly to the right of it. Switch between an aldose and a ketose by clicking on \"switch carbonyl group.\" Add or delete carbon atoms using the add (\" \") or delete (\"x\") buttons. Clicking on a blue box once adds a hydrogen atom (H). Clicking on a blue box again toggles between H and OH.