Here's why:
* Predicate: The predicate is the part of the sentence that tells us something about the subject. In this sentence, the predicate is "look tired today."
* Adjective: An adjective describes a noun or pronoun.
* Predicate Adjective: A predicate adjective modifies the subject and follows a linking verb.
In this case, "tired" describes the subject "you" and follows the linking verb "look."