You don't have to be an expert baker to create a tooth-shaped cake for your favorite dental student, hygienist or dentist. You do, however, need to use fondant, which is a bit more difficult to handle than spreadable frosting alone. The difference in appearance is worth it -- the smooth fondant will look much more like tooth enamel than frosting would. You can make your own fondant out of marshmallow or purchase it ready-made at craft stores and baking suppliers.
Place the sheet cake on a foil-covered piece of cardboard.
Cut around the sheet cake with a serrated knife to create a molar shape; search for a picture of a basic tooth shape to use as a guide. The shape is basically a large curve "m" on top with a smaller "w" on the bottom.
Spread a thin later of vanilla icing on the cake to seal the crumbs and to give the fondant something to stick to. It will be covered, so it doesn't have to look perfect.
Knead the fondant until it becomes soft.
Roll the fondant into a thin layer on a confectioner's sugar-dusted surface. The fondant needs to be larger than the cake.
Carefully lift the rolled out fondant -- have another person help, if necessary -- and lay it over the cake.
Press the fondant over the frosting all the way around the base of the cake, Trim off the excess fondant with a sharp knife.
Decorate the cake with icing, if desired. Use ready-to-use tubes of icing or put colored icing in a zip-top bag and cut a tiny hole in one corner.