Two teeth from Tyrannosaurus rex at different growth stages. The adult is a replica of a posterior right dentary tooth of Stan. The baby/juvenile is also a posterior tooth (about 6 mm long), found in the Hell Creek Fm. (more info: http://www.thefossilforum.com/index.php?/collections-database/chordata/dinosaurs/baby-t-rex-tooth-r1992/). It could also be a dentary tooth based on the wear patterns, but I'm not sure.