Follow Up to Earth, Flat, Hook, Sharp

In an earlier post I posed the following question: Earth, flat, hook, sharp. Which does not belong and why? As promised I am posting a follow up which contains my own answer to the riddle which I now believe to be wrong.

My theory was that Earth didn't belong because flat, hook, and sharp are all words that could be used to described the appearance or shape of an object. Take a look at the comments that follow the original post to find what I believe to be the correct answer as provided by my sister Hilary.

Earth, Flat, Hook, Sharp

Meredith's anthropology professor posed the following question:

Of the following choices, which does not belong: Earth, Flat, Hook, Sharp. Why?

I have no idea what the correct answer is or even if there is one. I will explain my own theory in a follow-up post. What's your answer?

Edited to Add: Formulate your own theory first before peeking at the comments!