Look at a cow, a dog, a cat, or an elephant. They all have ears you can see on the outside of the head. Animals with hair or fur on their bodies, called mammals, usually have outer ears that stick out. The outer ear is shaped like a cup. It catches sound waves travelling through the air and sends them down a tube to the eardrum inside the head. Many animals can turn their ears towards a sound without turning their head, which is why a deer or a rabbit can hear a tiger coming from behind.
Some animals hear well but have no outer ear. Birds, lizards, and frogs have only a small hole on each side of the head, often hidden under feathers or skin. A frog has a round patch behind each eye, which is its eardrum on the outside. Snakes have no ears at all. They feel vibrations through the ground with their jaw and body, so a snake charmer's snake is actually watching the moving pipe, not listening to the music. Fish feel vibrations in the water with a line of tiny sensors along their sides. Elephants have huge ears that not only hear but also flap to cool the blood inside them.
Example: Cup your hands behind your ears and listen to a quiet sound, such as a ticking clock or a whispering friend. Then take your hands away. Which is louder? Your cupped hands act like the big outer ears of a cow or a deer.