A dolphin's sonar is so powerful it can stun or kill small fish.
Why it is true
Dolphins emit high-frequency clicks that create pressure waves capable of disorienting or even incapacitating small prey before capturing them.
Source: Marine Mammal Science
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.