This specimen simulates the mouth of a human with two remained teeth. The teeth were grinded by a dentist (Cylinders). Now the grinded teeth have to be impressioned. To measure the contraction of impression materials you have to put the material into the tray, put it into the body and close the cover. After the material is untied, open the cover and remove the impression material out of the tray. Leave it some time and then measure it with a measuring stick, if the original distance of the teeth is remained or if the impression material changed length. And if so, how much ist the difference. So you can judge if the impression material is storable or not. To facilitate the measuring the tray is not curved as an original tray at the dentist. It is just a plain situation. So you can calculate the difference easily.