The two factors that affect the gravitational force between two  objects are Mass and Distance.   The mass of the two objects and the distance between them according to Kepler's Law Fg = GMm/d^2. Gravity exerts itself between all bodies in the universe. It is related to the mass of the bodies and the distance between them.
Note: none of these elements exists freely; all are found chemically combined with other elements in compounds. Although there are over 100 elements, only 12 or so are used to make biological materials. Living organisms are built predominantly from non-metal elements.
-- Living things contain DNA and/or RNA, proteins which contain the basic information cells use to reproduce themselves. Fire does not contain DNA or RNA. -- Living things are made of matter, and you can weigh them. It may be hard to measure the weight of one bacteria, but you get the point.