C: Why isn't size_t a C keyword?

Question

sizeof is a C keyword. It returns the size in a type named size_t. However, size_t is not a keyword, but is

Answer 1

From MSDN:

When the sizeof operator is applied to an object of type char, it yields 1

Even if you don't have stddef.h available/included and don't know about size_t, using sizeof you can get the size of objects relative to char.