It's important to understand the difference between a fact, an observation, and an inference before we can determine what "must be" one of them. Here's a breakdown:

* Fact: A statement that can be proven true. It is objective and based on evidence.

* Example: The Earth is round.

* Observation: Something that is noticed or perceived using the senses. It is a direct experience.

* Example: The sky is blue.

* Inference: A conclusion reached on the basis of evidence and reasoning. It is an interpretation of an observation or facts.

* Example: The grass is wet, so it must have rained.

What "must be" one of these things?

* A statement of a fact must be a fact. It can be verified as true.

* A description of something you see, hear, smell, touch, or taste must be an observation. It's a direct perception.

* An explanation of what you think happened based on your observations must be an inference. It's a conclusion you reach based on the evidence.

Important Note:

* It's crucial to avoid misinterpreting inferences as facts. An inference can be incorrect even if it seems logical based on the observations.

* Observations can be subjective, meaning two people might see the same thing but perceive it differently.

Let me know if you have a specific statement you want to classify!