As a biology teacher, I am just trying out Ask a Biologist with my first grade son to get a feel for how it works.  He would like to know how big are T-Rex's teeth, and are they all the same size/shape?

Thanks for helping us answer our question!

I have a (cast of) a T. rex tooth on the shelf right in front of me as a type this. The crown (the part of the tooth that would have been visible, projecting above the gum) is about 15 cm (6 inches) long. The root is truncated in my cast, but probably would have been about the same length, making the whole tooth about a foot long.

Not all the teeth of this individual would have been the same size or shape. In general, the teeth a relatively small right at the front of the mouth, then larger, then become progressively smaller towards the back. But there are lots of exceptions to this general trend, dependent on how old each individual tooth is. (Teeth were replaced continuously throughout life, so a big tooth could be pushed out by an initially small replacement.)

They're not all the same shape either. The front teeth are D-shaped in cross-section, while further back the cross-section is more like a rounded diamond.

And most important: tyrannosaurs did not have sharp teeth! The common description of them as "like steak knives" is completely wrong: they were more like rock-solid bananas. Why? Because they optimised for crushing rather than for slicing. Tyrannosaur teeth are hammers, not knives.