Standard input is empty

Polar Bear, Polar Bear, what do you hear?
I hear a lion roaring in my ear.

Lion, Lion, what do you hear?
I hear a hippopotamus snorting in my ear.

Hippopotamus, Hippopotamus, what do you hear?
I hear a flamingo fluting in my ear.

Flamingo, Flamingo, what do you hear?
I hear a zebra braying in my ear.

Zebra, Zebra, what do you hear?
I hear a boa constrictor hissing in my ear.

Boa Constrictor, Boa Constrictor, what do you hear?
I hear an elephant trumpeting in my ear.

Elephant, Elephant, what do you hear?
I hear a leopard snarling in my ear.

Leopard, Leopard, what do you hear?
I hear a peacock yelping in my ear.

Peacock, Peacock, what do you hear?
I hear a walrus bellowing in my ear.

Walrus, Walrus, what do you hear?
I hear a zookeeper whistling in my ear.

Zookeeper, Zookeeper, what do you hear?
I hear children...

...growling like a polar bear,
roaring like a lion,
snorting like a hippopotamus,
fluting like a flamingo,
braying like a zebra,
hissing like a boa constrictor,
trumpeting like an elephant,
snarling like a leopard,
yelping like a peacock,
bellowing like a walrus...
that's what I hear.