TITLE DIGITAL STUDIOS The American poet Ogden Nash said "happiness" is having a scratch（抓） for every itch（发痒）. But while scratching can feel pretty good, we all know what happens next. When considering scratching an itch, it may be better to follow the old Yiddish proverb. A quarrel is like an itch. The more you scratch, the more it itches. There are plenty of things that cause that irritating, unpleasant, itchy sensation（感觉）.

  But why do we itch in the first place? On a blueprint（ 蓝图）, TITLE okay to be smart. The itch sensation, also known as pruritus, is not the most pleasant thing. But it evolved（发展） for a good reason. Stretched（伸展） out, your skin covers about 20 square feet, the biggest organ（器官） you have. Your other organs on the inside are protected by your immune（免疫的） system.

  But skin is your first line of defense, the wall that guards the human realm（领域）, exposed（使暴露） to the elements, and it's got a unique（唯一的） way to deal with different types of threats（威胁）. Where there's an itch, there's a desire（愿望） to scratch（抓）, often an unconscious（无意识的） desire（愿望）. This reflex likely prompted（敦促） our outdoor dwelling（住处） ancestors（祖宗） to remove dead skin or parasitic threats（威胁）, and other irritating things like thorns（刺） and allergens. And much like yawning, being itchy is thought to be contagious, as some of you may be noticing right about now. We're social animals, so watching some infested（大批滋生） tribe（部落） mate（助手） get their scratch（抓） on could have given us a head start to get rid of those parasites. So we know the purpose of itch, but what happens inside our bodies that makes it feel different from other sensations, like pressure or heat?

  Turns out we don't know all the pieces to the itch puzzle（难题） yet. Up until about a decade ago, scientists thought itch was just a dialed down, less intense（强烈的） version of pain. When something makes contact with our bodies, nerve（神经） endings, and the epidermis（[生]表皮）, the outermost（ 最外面的） layer of our skin, relay this information through electrical and chemical messages up the spine（脊柱） and to the brain.