Cusp to root, the cutaway model splits open every time a hygienist needs to explain a filling, the hinge lifting the top half to reveal, in cross section, four colors of plastic standing in for enamel, dentin, pulp, and a root canal painted a pink no actual nerve has ever been. The gimmick is decades old and essentially unchanged: a molar, always a molar, enlarged to roughly the size of a fist, ranging from white through yellowed cream to amber-brown depending on age and how many hands have gripped it while pointing. Some models split into two pieces entirely, top and bottom separating at the gum line; most hinge on one side and swing open like a small book made of teeth.
The hinge is the first part to fail. After roughly one demonstration a day in a busy practice, the pin wears loose and the two halves no longer sit flush, so the model hangs slightly open even between demonstrations, a permanent half smile no dentist has bothered to fix, since a model that will not fully close still communicates a cavity just as well as one that will. Patients rarely touch it themselves; it is handed across the counter and turned in the hygienist's hand rather than the patient's, angled toward whichever tooth is under discussion. A few practices keep two, one healthy and one visibly decayed along a painted brown seam, alternating between them depending on the news being delivered. Nobody has tracked how many patients remember the model a year later, versus how many remember only the tooth it was standing in for.