The second sentence states a fact.
Rattlesnakes use their rattles to warn other animals.
It can be proved by looking up information about rattlesnakes.
The first sentence states an opinion.
Rattlesnakes are the most dangerous reptiles.
Most dangerous shows what a person believes, thinks, or feels. Another person might have a different opinion about which reptiles are the most dangerous.
Which sentence states a fact?
A: Rattlesnakes are the most dangerous reptiles.
B: Rattlesnakes use their rattles to warn other animals.