Here's why:
* Punctuated: Sentences always end with a punctuation mark (period, question mark, exclamation point).
* Group of words: Sentences need more than one word to convey a complete thought.
* Express a complete thought: A sentence must have a subject (who or what the sentence is about) and a predicate (what the subject is doing or being).
For example:
* The cat sat on the mat. (A simple sentence with a subject and a predicate)
* What time is it? (A question, still a complete thought)
* Wow! That was amazing! (An exclamation, still a complete thought)