Every animal collects information about the world through its sense organs, and many animals have senses far sharper than ours. A dog has about 300 million smell receptors in its nose, while a human has only about 5 million, which is why a dog can find a lost person by the scent of a single footprint. An eagle flying 3 kilometres high can spot a rabbit moving in the grass, because its eyes have many more light-sensing cells than ours. A cat sees well in dim light thanks to a mirror-like layer at the back of its eye that reflects light a second time. Bats and dolphins use echolocation: they send out high-pitched sounds and listen to the echoes to find their way and their food.
Some animals sense things we cannot sense at all. Bees see ultraviolet patterns on flowers that guide them to nectar. Snakes such as pit vipers have heat-sensing pits on their faces that detect the warmth of a mouse in total darkness. A shark feels the tiny electric signals made by the muscles of a fish hiding under the sand. Mosquitoes find us by the carbon dioxide we breathe out and the heat of our skin. Scientists say an animal that is hunted, such as a deer, has eyes on the sides of its head to watch all around, while a hunter, such as a tiger, has eyes facing forward to judge distance before it pounces.
Example: Test your own sense of touch. Ask a friend to close their eyes while you gently touch their fingertip, palm, forearm and back with two pencil points held 1 centimetre apart. Ask whether they feel one point or two. Prediction: the fingertip will feel two points but the back will feel only one, because fingertips have far more touch receptors.