Why do we receive on the tongue?
In order to understand this requirement, one needs to view it in context of the great care the Extraordinary Form of the Roman Rite takes to treat each Host with respect. In the Extraordinary Form, only ordained priests and deacons are allowed to touch the Host with their hands. While they are distributing Holy Communion, servers hold a plate called the paten under the mouths of those receiving in order to catch any particles of the Host that may fall. After the ministers are finished distributing Holy Communion, these priests and deacons are expected to purify their fingers and patens so that no particle may accidentally fall into profanation. Because of the great care required , it is impractical to have the laity receive in the hand even though this is common practice in the Ordinary Form. We also only receive the Sacred Host and not from the Sacred Chalice because each Host already contains the complete reality of Christ's Body and Blood.