Blood

Blood is a somewhat thick substance that comes out of injuries. It has to be in the body, and if you hemerage (lose all of your blood) then you will die.

In folklore, this is what vampires thirst for. They traditionally suck the blood of a young human, and then that human turns into a vampire. However, in the Twilight Saga, sucking human blood can be avoided, and even when a vampire does, it simply kills the human, not turn them into a vampire. The transformation from human to vampire is much more painful than this.