Taking an impression involves placing a putty-like material in a mouth tray and positioning the tray over the teeth. It is held in position for about a minute, until the material hardens to the consistency of Jell-O. An exact copy of teeth is obtained in this way. This is then taken to the lab where liquid plaster is poured into it. After the plaster has hardened, the tray is removed and a duplicate of your teeth remains in the plaster models. This is called taking a mould of your teeth.