Blood

"Blood is about to spill, all right. Yours!"

- General Grievous to Darth Maul

Blood was a liquid substance present in most sentient beings though not all species had the same color of blood. Humans and Aqualish, for example, had red blood while Rodians, Duros, and Trandoshans had green blood. Kiffar had scarlet blood. By testing a sample of someone's blood, it was possible to get a midi-chlorian count and determine how sensitive one was to the Force.