If you hit your head against a tree all day long，you probably wouldn't feel too great and neither would your brain.Woodpeckers do this all the time.
Some animals are human's best friends.The woodpecker is one of them.The bird helps trees to grow by eating worms inside the tree trunk.They find worms by hitting the tree trunk.They hit their heads into trees with a force of 15 mph （24 km/h），20 times a second!So why don't they get headaches？
The answer is in their brains.
The texture（质地）of a woodpecker's brain is different from a human's.Our brain is filled with cerebral spinal fluid（脑脊液）.When we hurt our head，our brain moves around in the fluid.But woodpeckers have none of this fluid.This means that when they hit the tree，their brain is less likely to move around.
Besides，thick muscles and sponge﹣like bones（海绵状骨头） also help to protect the bird's brain.They keep the brain from moving around when the bird is working on a tree trunk.
The fast and strong hitting may also harm the woodpeckers' eyes.Don't worry.The birds have a special trick.They close their thick inner eyelids（眼睑）when hitting the tree.The eyelid acts like a seat belt and keeps the eye from getting out of the head，or the speed would tear the eye.
Scientists even think some of the things they found out about woodpeckers can be used to make better helmets（头盔）.
①the texture of their brain
②the cerebral spinal fluid
③thick muscles
④sponge﹣like bones
⑤their inner eyelids
