This picture of a narwhal is from https://en.wikipedia.org/wiki/Narwhal
Recently my son keeps on talking about a narwhal, which is a medium-sized toothed whale with a very unique looking. A male's (occasionally female) tooth/tusk (sometimes teeth/tusks) looks like a horn and is used to exert dominance. Like other whales, a narwhal is a mammal, and exhibits seasonal migrations.
I asked him where he heard this from, he answered that he saw it on a cartoon show called Wild Kratts on PBS Kids (referral link).
Here are the dialogues with kids:
A = Adult, S=Son
S: Hey, do you know what is narwhal?
A: No, I do not know.
S: I know. It is a kind of whale usually has one tusk and occasionally has two tusks.
A: Wow, I don't even know a whale can have horn(s)! Can you tell me more details?
S: They eat fish and they are mammals.
A: Do you know what is mammal?
S: Not really.
A: Mammals mean females produce milk to feed babies. Do you know how are birds born?
S: The ugly duckling came out of an egg!
A: Can you guess whether birds are mammals?
S: I do not know.
A: Do birds feed milk to babies?
S: No, they feed babies worms.
A: So birds are not mammals.
S: Do you mean female narwhals feed milk to baby narwhals, just like us?
A: Yes. Is that cool?
S: Yes.
A: Let me search for narwhal to find out more.
(After I checked wikipedia about narwhal...)
A: Do you know what do narwhals do with the tusk(s)?
S: They fight with the tusks.
A: Yes, it is true, but it is only one function of the tusks. There are a lot of nerves in the tusks so that narwhals can use tusks to connecting seawater stimuli with the brain. Plus they also use tusks to stun small fish for easier catching. However, since most females do not have tusks, tusks care not critical parts of narwhals.
S: Wow.
A: Do you know that they also exhibit seasonal migrations?
S: No, tell me more.
A: They move to offshore in the winter and move closer to coasts in the summer.
S: Well, it sounds like normal whales.
A: Yes. Searching on web is your good friend for learning new knowledges, like always.
S: I know.