Explain what exactly? From the image name, I am guessing that Ms. Mueller would like to know why this turtle appears to have teeth. Some turtles have spines in their mouth and esophagus called papillae, which can resemble teeth. These are not used for chewing, but rather to prevent food from being expelled when excess water is pushed out before the turtle swallows.

The best example seems to be the loggerhead sea turtle.

All examples of turtles with papillae that I found were sea turtles though, not ones which live in Missouri. There are other references to structures called papillae in turtles (examples here and here). These are used for respiration, but seem to be distinct from the tooth-like papillae described above; the respiratory papillae may be described as lobe-like and usually on the tongue, not on the roof of the mouth.

Good question. My guess is that it is a developmental mishap resulting from the genes that create teeth being activated in this particular turtle. This is a phenomenon known in other normally toothless animals, too; for example, there is a gene in chickens that can be activated and will cause the chickens to grow teeth. I don't know for sure, but this seems most likely.