A leech in its neutral position has a length and thickness of a match. After some time of being stacked to the skin of a mammal, it fills with blood and thickens to the size of a finger. Usually their bite is undetectable and they suck the blood of its unaware host. After feeding, they fall off and their active anticoagulant (hirudin) in their saliva prevents the blood to clot for another 20 min. causing prolonged bleeding.